After adding this pack to your launcher you must open its settings dialog menu (the little gear on the bottom left of the pack icon) and select your preferred version of Forge/MC. After setting or changing the Forge/MC version simply start the pack once and it will be ready for your testing your mods/configs/packs.
Some of the option "builds" have suffixes listed. This is what they mean:
_LL - The modpack.jar has LiteLoader pre-configured.
_PG - The modpack.jar has the PermSpace/PermGen settings increased (useful for large packs, not relevant for Java 8+).
_LL_PG - Both of the above.