With the Gerber USB to serial adapters, you have to go into the device manager and see what port number it is assigned to, then in the Add plotter box, make sure that you're selecting the correct port. If the port number doesn't show as available in the Add plotter box, go back to the device manager and look under the advanced settings. You should be able to change the assigned port number there.
Many people try to set up the plotter as communicating via USB in Gerber - it doesn't work that way. Select the appropriate com port and you're good to go - this applies to any USB to Serial port adapter (for plotters without USB connectivity) that I've seen out there. And (almost) without saying, load the driver from the Omega disk or download from GSPinc.com.
The USB to Parallel adapter (for Edge connectivity) is different, you need to tell the Add vinyl printer dialog that you are connecting via USB and then go into the properties of that set up and make sure that you select the correct USB settings and make to click preferences on you way out.
Hope this helps and Happy New Year to all.