To build a little on jajks answer (love those answers BTW. No bullshit. Straight to the point lol):
Get Forge Installer (version .916 works for me with MC1.6.4): http://files.minecraftforge.net/
Install Forge. Run MC. Stop MC.
Now put RiM in the %appdata%/.minecraft/mods/ folder (If on Windows, that is)
Run MC. Enjoy.
If you need more help than this use google. Tons of tutorials out there.