the px4 wasn't valid hardware when 2.68 was released. so no firmware for px4 exists.
When you said "no firmware for px4 exists" means that there is no firmware for px4 to load in a px4 with mission planner?
if you load the 2.74 there is a px4 firmware, but the original question was about 2.68
current MP version is 1.2.84 and arduplane 2.74