To access Ground Assault for the Separatists, you must be on the Invisible Hand and have collected enough Gold Bricks to open the ship located in the main hangar bay. Once through, there should be a control console on the main deck of the ship that will allow you to access that console.
Buy a heavy vehicle to blow through the defenses and there you have it. Note that all planets in the system must be converted to Separatist (Red) at the same time in order to unlock the achievement.
While working for the 100% achievement, once every system has been converted to Separatist once, you will unlock Grand Moff Tarkin as a playable character. Note that if a system begins under Separatist control it must be converted to Republic and then back to Separatist in order to unlock the character.