do you actually have to tell the target (out loud or in his mind) what he is suppused to do? no mention is made of language, so i would guess no.

is the target aware of being compelled (or does it perceive his actions to be it's own decision?)? if the power works? if it doesn't?

It is part of the telepathy discipline, so no spoken words are needed.

I've ruled in the past that a character of sufficient intelligence will notice he's being compelled to do something, though he might not realize the source.

A particularly stupid person or an animal won't notice the difference between the effects of Compel and their usual instincts/whims.