in case the "valve" talk didn't make sense to you, many inground pools will have shutoff valves for one or both of the return lines - the one from the main drain, and the one from the skimmer. the valves would probably be close to the pump entrance. make sure that both valve handles are turned parallel to the pipe.

i just opened my pool yesterday, and i would say the pump ran for close to a minute before there was any water at all running into it, and a good two minutes until it was totally primed.

what you're looking for is the point where you can't see any air bubles at all inside the pump - it should be completely filled with water. if that doesn't happen after a couple of minutes, report back.

also, make sure that the returns into the pool are unblocked. they may have had something put on them for winterization.