what kind of error message are you getting?
make sure you follow the guidelines here:
http://support.apple.com/kb/HT2084?viewlocale=en_US
note: your router either has to support NAT-PMP or UPnP
following ports have to be enabled:
TCP 443 (signing into MobileMe and enabling Back to My Mac)
UDP 4500 (for connections between machines)
Back To My Mac has been hit or miss for me... just been easier for me use VNC/ScreenShare (with a SSH tunnel).
cheers...