Turns out many D-Link DSM-750s are “defective” and will need to be sent back to D-Link to be fixed by a “factory-trained technician” because of an “issue with a corrupted security certificate" (and possibility because the firmware they shipped was from 6-months ago).

If you have a DSM-750 and your attempts to connect it as an Extender to a PC running Vista Home Premium or Ultimate result in a “Communication Error” then you can call 888-900-3939(Monday-Friday from 9am-5pm PST) to get in direct touch with D-Link CSR.