You do not need to download anything in order to play the game cooperatively. However, you will need to move some files around. Here's how to do it.

1. Locate your Aliens vs. Predator folder. You can do this by right-clicking the game in your Steam library, go to Properties, click on the Local Files tab, then click Browse Local Files. The file directory should be C:\Program Files (x86)\Steam\SteamApps\common\Aliens vs. Predator.


2. Open up the Envs folder.


3. Find the single-player level you want to play. The levels are called Colony, Jungle, Lab, Pred_Tutorial, Pyramid, Refinery, and Ruins. All species play the same maps. You can tell them apart based on whether they begin with A (for alien), M (for marine), or P (for predator). The numbers after the letter tell you what stage of the campaign it is for that particular species. For instance, the Lab level for the alien is called A01_Lab (A for alien, 01 because it's the first map in the alien campaign).


4. There are three files for each single-player level. Once you've found the level you want to play, COPY those three level files. They should end in .eng, .lst, and .pc. We'll use A01_Lab as an example.


5. Go back to your Envs folder and open up the MP folder.


6. PASTE the three level files into this folder.


7. Now choose the multiplayer map that you want to replace. This doesn't really matter, but it's the level you'll be choosing in-game for the multiplayer server. Let's use Gateway for our example. Ignore the file that ends with extension .hrd. Retitle the other three Gateway level files to MP_00_Gateway_ORIGINAL.eng, MP_00_Gateway_ORIGINAL.lst, and MP_00_Gateway_ORIGINAL.pc (Alternatively, you can cut and paste them into a backup folder).
Do NOT rename the file that ends in extension .hrd.


8. Now retitle the three campaign level files that you just pasted (A01_Lab in our example) to MP_00_Gateway.eng, MP_00_Gateway.lst, and MP_00_Gateway.pc


9. You're done. Now when you select that particular level that you replaced in multiplayer, you will be able to play the campaign. Your folder should look something like this:


You can repeat the process for whatever campaigns you like. Just keep a mental note of which multiplayer levels you replaced with which single-player ones. Make absolutely sure that the other person/people with whom you're playing do the exact same thing as you, or else you may not be able to play together.

Once you and whomever you're playing with have moved the level files to the MP folder, start the game and go to multiplayer. Go to Player Match. Choose Create Game (if you're the host). Choose Death Match or Mixed Species Team Deathmatch (bots will kill each other in Deathmatch but not in MSTDM). Pick the level you chose to replace (in the example above, we replaced Gateway). Set the score limit high and time limit to 60 minutes. Get your friend(s) to join and when you start it, you should be in whatever single player campaign map you copied and you can play cooperatively against the A.I.


PLEASE NOTE!
Many of the levels will give you trouble. Some doors will permanently lock behind you, making anyone who's behind locked out of the next part of the map. Other doors that are imperative for progressing simply do not open. Some textures can be clipped through. The reason for these problems is because these levels were not intended to be played cooperatively. Play at your own risk.

The fact that you're playing on multiplayer will also change some gameplay elements. For instance, because the predator only starts out with the wristblades in multiplayer, when playing these levels as the predator in coop, you will not start out with weapons like the disc, combi-stick, shoulder cannon, or bombs as you do in the campaign.

None of these problems will break the game but it may make some levels impossible to complete. Please do not leave comments telling me that I broke the game or asking how to get around a certain level's problems. For the former, it's simply untrue and I've stated the reason why some levels can't be completed; for the latter, I simply do not know.

I highly recommend moving back any moved game files after playing coop. I especially do NOT recommend choosing "Continue" after a coop session, as AVP will get horribly confused as to where you are, and you'll likely need to cntl-alt-del your way out of the game.

This guide was written solely by me (Olde). However, it would not have been possible without the extremely helpful guide by john14b. It's amazing some of the things you can learn with Google Translate.

You can (and should!) use this guide and share it with others. However, you should not distribute it while giving authorial credit to either yourself, someone else, or to nobody at all. You may quote from it at length but please do not post the entirety as a guide on any other forum or site without my express permission. I also reserve the right to remove or modify this guide at any time and/or make it unavailable, with or without providing a reason.

I hope you enjoyed this guide! I value your input so please leave a comment below! Thanks for reading!