I'm new too and having the same problem. I think it has to do with the fact that my host has only USB 3.0 ports accessible. When XP boots in Player the "New Hardware" wizard appears, having found a USB controller. It cannot find drivers and ultimately gives up. The non-functional controller is listed in XP's Device Manager under "Other Devices" rather than under "Universal Serial Bus Controllers". Anything I plug in gets grabbed by the host whether the VM has focus or not.
My mouse is plugged into the other USB 3.0 port and works fine in and out of the VM, but it must be handled in a special way; note that it appears in the VM as "VMWare Pointing Device"
Looking at the drivers listed in the host's Device Manager it appears that my USB chipset is Intel and drivers for it are not available for XP or Vista. If you have something else, say NEC chips, you might try installing 3.0 drivers in XP.
My setup: host = Windows 7 Pro (x64); Player = 5.0.1; guest = Windows XP pro, imported from XP Mode.
I need a hub anyway, so I think I will buy one that has both 2.0 and 3.0 ports. It's a long shot, but just maybe...