This same list shows what mods you're currently using and their load priority.
If a mod wasn't unpacked before, clicking Apply will unpack it and make it ready for use by the game. To activate a mod, simply tick the little checkbox on the left of a mod name in the list and click Apply. Available in the sense that they're downloaded and copied into the InstallationFolder\Mods.
Open the Game Launcher -> Mods window to see the list of available mods.
