hello there !
First, you must download Freelancer Mod Manager. I suggest that you should read the mod’s comments and requirements. some mods needs a clean FL mod manager 1.31, or 1.4 beta 4.
when you Install the FLMM, run it as administrator and double click to the mod. manager will activate sellected mod. then run freelancer 😄

Also, some other mods does not need a freelancer mod manager installation. for example, discovery freelancer doesnt need flmm. just download and install it.