The lovely Molly McDonald from demogirl.com has recorded an excellent screencast about MindMeister where she talks you through the entire system, starting by building a small map and going on to advanced features such as sharing, notifications and tagging.
The screencast is four and a half minutes long and in high resolution with a voiceover by Molly. It is currently only available in Flash format (yes, there is apparently one part of MindMeister that you need Flash for 
) so you will need to download the player from here if you haven’t already done so. We’ll ask demogirl if we can host a version on YouTube as well.