You need to enable the mods you want to activate, before the game will be able to read and use them.
This is done via the Launcher, and there's two ways to do it.
First is to open Launcher.exe and go to MOD tab; then click "Select MOD" and navigate to appropriate folder inside your FS2 installation directory.
Second is to do things via custom command line option on the Features tab, with the
-mod mod_folder1,modfolder_2,(...),modfolder_N, with which you can manually activate as many mods as you wish (there's probably some limit to the number of mods but I haven't found it yet...). Mod folders are separated by comma.
The first option is easier but doesn't give the same level of control, and things like making sure that mediaVP's are used alongside any mod are sometimes a bit of a pain in the neck, but for starters that is a good option.
The second option is, like said, a direct route to configuring the data the game will use, and it also is basically the manual version of the MOD tab and mod.ini files, nothing more... but it also offers a way to make things unwork by activating several conflicting mods at a time. Generally, it's best to activate just one mod and mediaVP's at a time, like
-mod Derelict,mediavps
Incidentally, the custom command line is a very useful tool when you learn to use it. Be sure to take a look at
Hard Light Wiki Command-line Reference at some point, you'll probably find it... enlightening.
