I am having the same problem, but its intermittent. Last night I was able to connect MMC to the designated computer, but today I am not.
Its driving me nuts. As well, when I start MCC on the computer, it then launches WMP (Windows 7), and a notification pops up saying "it may take a while before you can connect" (or something like that). It was doing that last night but I was able to immediately connect, but today I am not able to connect at all. No configurations have changed except that I blew away WMP Library and reloaded it. I waited until the Library was fully loaded then tried connecting with MCC and it just sits there forever trying to connect.
I restarted all apps, I have rebooted phone and computer and it still won't work.
I have since tried removing Authentication passwords and "forcing network connection" - nothing is working.
Phone: HTC Touch Pro II, mobile 6.5
Computer: Windows 7 SP1
MCC: v1.5.6