First, are you sure your map was made for Rebellion? Old maps are sadly no longer compatible. To check, just open it up in notepad and see if it says "Version 3" somewhere near the top.

If it says Version 4, go to "My Documents", "My Games", "Ironclad Games", "Sins of a Solar Empire Rebellion". You might want to make a shortcut to this folder as it holds the mod folders, save folders, replay folders and most other content you might be interested in. Place the map in the folder called "GalaxyForge". This location was changed during the Rebellion beta, so thats why most advice was out of date.