Both ends of USB cable connected firmly? Printer turned on? 🙂
Have you tried a different USB cable?
Also -- you've said MacPro in one spot, Macbook Pro in another. They're different machines, so it might be worthwhile to clarify which one you're talking about -- though it shouldn't really matter.
As nunes013 asks, what is the printer and did it come with any drivers on a disk or downloaded from the manufacturer's website?
It'd be best to give more detail as to what you're doing and what the setup is; your question as asked isn't much different from telling a mechanic "my car doesn't work, what's wrong?" 🙂