Do you have the FSO Launcher (called Launcher.exe) in your FS2 folder?
Do you have the .12 builds from the .12 release thread unpacked and placed in your FS2 folder?
Do you have the mediavps in a folder called "mediavps_3612"?
If the answer to all questions is yes, then open the Launcher. Please select the fs2_open_3_6_12r exe in the Launcher by using the "Browse" button. Then use the Launcher's "Features" tab to enable the graphical enhancements. Next, click on the Mod tab. Select the 3.6.12 mediavps as the active mod.
Once that's done, click on run.
Should this be unclear, please join the #hard-light IRC channel. Link can be found on top of the page, or in my signature.