In your options\mods there is a button that says "show mod path" Click it, and it will show you where to install mods for Rebellion. It will also create the mods folder itself if it doesnt exist yet.
You will need to convert the Rebellion GameInfo files to text format using Harpo's textbinner tool. There are reference files available for Rebellion, but they are from version 1.01. The current version of Rebellion is 1.02, and will soon be patched to 1.03. So some of those reference files may be obsolete. He may post updated reference files later when rebellion gets patched.