I have the same issue. My laptop can read/write/modify my desktop with no issues at all. Yet WiiMC will not do this. The weird part is if you share a folder it works fine. Its stupid and doesn't make sense I think its a bug somewhere.
I cannot share anything under Windows 7 without the homegroup enabled. Its really annoying, but maybe that is what is causing these issues with WiiMC. It seems as though the emulators, FCEUGX and SNES9XGX is doing the same thing. I gave up, I'm going to just copy to my NTFS partition if I want to watch anything.