Installing Mods for FS2 Open
Improvements upon Retail
While installing mods for FreeSpace has never been hugely difficult FS2_Open has made considerable improvements to the way that they are handled. Under retail installing more than one mod involved either installing several copies of FS2 or tiresome dragging and renaming of folders and files. Under FS2_Open less than a minutes work when installing a mod can mean that you can swap between mods with nothing more than a click of a button.
Imstalling the Media VP files
To truly take advantage of FS2_Open's modding features you should install the media VP files to a subfolder of your Freespace2 folder. Although you can call this folder anything you like the assumption that will generally be made is that if you have put them in a folder it will be called MediaVPs so it is probably best to stick to that. Now that you have put the media VP files in that folder you must go to the mod tab on the launcher and select MediaVPs as your mod when running FS2. If you don't do this FS2 will run with none of the media VPs features. It will be like playing retail again.
Installing the mod
Be very wary of what the readmes that come with mods tell you to do. Many mods were written before FS2 or FS2_Open had these new features or by people only vaguely aware of what they do. For this reason I'll give you a golden rule.
NEVER install anything to Freespace2\Data regardless of what the instructions say.
If you are following this guide there is no reason to install there (with one exception I'll come to later). Installing to that folder means that FS2 retail and multiplayer will load the mod and probably break. The only exception is if you want the mod to work on multiplayer or retail. For instance if you have downloaded the validated multiplayer missions then there is no harm in placing them there as you want retail multiplayer to be able to access them. However you must take great care when doing this. Any problems you experience later could very easily be due to you installing a retail mod and then forgetting you did it.
Since FS2_Open will run the vast majority of retail mods you're probably better off simply installing the mod for FS2_Open.
What you should do is make a folder within your Freespace2 folder. Make sure that this folder has no spaces and no hyphens (In fact FS2_Open doesn't like it if there is a space anywhere in the path so C:\Program Files\Freespace 2\ModName\ is right out). If the mod consists completely of VP files stick them all in there and you're done with this step.
If the mods readme tell you to install to Freespace2\Data make a new folder called Data within the one you just created and then follow the instuctions using this new Data folder as if it were Freespace2\Data (i.e if told to stick a file in Freespace2\Data\Tables you stick it in Freespace2\ModName\Data\Tables).
The last step is only required if you want the mod to be able to access the media VP files. For most FS2 based mods this will be true. This is done by creating a new text file called mod.ini (be careful that the computer doesn't call it mod.ini.txt). Cut and paste this into the file
[multimod] secondrylist = ,Media_VPs;
Place this file in the mods folder. Run the launcher, click on mod, select the new folder and you're ready to play.