If you have never tried to install mods before, it is clean. Otherwise, backup your important stuff such as saves and screenshots, delete and run Minecraft
Run your profile as Version 1.6.4 and exit Minecraft
Download the Forge Installerhere (1.6.4 Recommended Build, Version 9.11.1.953)
Run the installer for your client
Open Minecraft and run the Forge profile and exit Minecraft
Download the Biomes O' Plenty Universal (1.6.4 Recommended Build, Version 1.1.2.51 (Watch out for misleading ads)