Is this posting through Habitat talk or through something like You-tube? If your trying to post through Habitat talk I don't know if you're trying to use You tube or something similar here's one thread from awhile ago.
Only way I have been able to do it is to load the video on to a video hosting site (like you tube) and then post the link/address to it here. Video file formats tend to be some strange ones sometimes....and video files are HUGE.