Ok, I got some help from Laura. I'll share the info here, so it could help others in the same situation. So here's some AGSteam Linux installation instructions:
1. Go to linux32 folder in the installation package, and rename libagsteam-32.so to libagsteam.so
2. Place the 32 and 64 bit versions of libagsteam.so and libsteam_api.so in the Linux subfolder of the AGS engine folder (Linux\lib32 & Linux\lib64)
+ Place the files also in compiled/linux/data/lib32 and lib64 folders (not sure if this is necessary, but just in case)
And that's it (as far as I know)!
1. Go to linux32 folder in the installation package, and rename libagsteam-32.so to libagsteam.so
2. Place the 32 and 64 bit versions of libagsteam.so and libsteam_api.so in the Linux subfolder of the AGS engine folder (Linux\lib32 & Linux\lib64)
+ Place the files also in compiled/linux/data/lib32 and lib64 folders (not sure if this is necessary, but just in case)
And that's it (as far as I know)!