Here's some info from Mark Espenschied, a Marketing Communications Manager for D-Link Systems, Inc. in Fountain Valley, California.
" A small number of early production DSM-750 Wireless N HD Media Center Extenders have an issue with a corrupted security certificate, which results in the communication error messages described in this post when trying to affiliate the DSM-750 with a PC running Windows Media Center. Unfortunately, this defect cannot be rectified through firmware upgrade, and can only be addressed by a factory-trained technician.
If you receive the communication error while attempting to affiliate the DSM-750 with Windows Media Center, please call D-Link technical support at 888-900-3939, Monday-Friday from 9am-5pm PST. This a special number which will allow you to directly reach appropriate technical support agents. They will assess the issue and determine if your unit needs to be replaced. D-Link is ready to quickly resolve this issue; tech support agents have been briefed and customers will be shipped replacement units if needed.