Um..The map you download must be in .rar form, I also presume your using windows 7 so...Your legions game folder should be on your hard drive " Local disk" or wherever you've placed it.Open the downloaded map file then look at what folder you see..You will mostly see "Mission", "Skies" , "Terrain", etc.Now switch back to the legions file on your "Local disk".Once you've done that just go into "Legions Overdrive" and then follow the instructions below :
Legions Overdrive >Public test>(If the downloaded file has missions then) > Legions > Data> (Then you placed the "mission" on the .rar (downloaded file content).Into the mission folder and your done.If the downloaded folder also has Skies and such to add in.You will find it in the same place where you found missions.