The PC-50 normally requires that a Roland USB driver be installed on your computer to be able to communicate correctly. If you have an operating system that is not supported, you can set the PC-50 to use the MIDI driver that is built into your computer's operating system (for instance, Core MIDI on Mac OSX). This is often called "Class Compliant."
To make this setting, change the Advanced Driver switch on the side of the PC-50 (by the MIDI output jack) to the "OFF" position. This will allow you to connect the PC-50 to a computer (Mac or PC) without the need for a Roland USB driver.