Instructions
Open your "Combat Arms" directory. This can be located anywhere on your computer, depending on where you installed the game's files, but the directory is located in the "Program Files" folder by default.
Open the folder where the cross game mod will be be placed. Almost without exception, the required installation location for a mod is listed in the mod's description or in a file called "Read Me" that comes with the mod. The mod will have to be placed into the correct folder to function properly.
Drag the cross-game mod into the required location. A selection window for Windows will appear notifying you there is another file of the same name present in the folder.
Click "Copy and Replace" to install the mod. Since "Combat Arms" doesn't have a way to directly activate a mod, it will have to overwrite whatever file it is currently a mod for. For example, a mod that changed the look of the machine gun will have to overwrite the current machine gun texture.