[Fyron]

Actually, in SE4, any time you use the colonize order while orbiting a planet, population will be loaded. There is no need to use the planets screen for this to happen.

Jeku:
If your colony ship is orbiting a planet and you issue a colonize order directly, SE4 will add a "load population" order just before the movement order. If you move the ship out of orbit though, no population will be loaded. This is what is happening. Make sure to transfer population to the colony ships before sending them out. To fix the 0 pop planets, you can just send a colony ship with population on it to them, then use the transfer cargo order to transfer 1 million (or more) pop to the planet.