Hot air rises, cool air sinks. If there is heat in your house, it's going to end up upstairs. When your unit is in your basement, it's got to work that much harder to get to the second floor. The air vents on both floors have adjustments for how open or closed they are, and it's just a matter of pressure balancing. Close the first floor vents slightly, and maybe even a fair amount if you're ever cold in the areas near them. As was said the basement is also almost always over-cooled, so close or nearly close all those vents.
I know somebody else kind of suggested this, but I figured I'd explain it a bit better. I intern for a large building controls company that deals heavily with hvac, so I'm exposed to it a lot. In larger buildings this process is called "air balancing" and is pretty time consuming and complex.
Edit: You can also close or nearly close vents in rooms that generally aren't used or have the door closed, and check that none of your vents are blocked by furniture or anything.