To expand on what skylight posted regarding non FTDI chipset adapters, you should check to see if your PC recognizes the USB serial adapter when it is plugged in. Go to the control panel and check to see what if any serial ports are available before you plug in the adapter, exit control panel, plug in the adapter, and check again. If the USB to serial adapter is recognized you should have one more com port than you did on the first check.
If there is no additional com port you may have the wrong driver, an incompatible serial adapter, or a defective serial adapter.
Agreed, it's most likely a USB adapter problem. Nothing unique about a Scribbler robot, it should show up as a standard BS2, so search the forums for resolved issues with your particular adapter.
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Your adaptor may not use the FTDI chipset?
If there is no additional com port you may have the wrong driver, an incompatible serial adapter, or a defective serial adapter.