To add a tag to your tag cloud, you have to actually use the tag on one of your own clips. Your tag cloud is different from everyone else's. On your page under top tags (which are your tags, and not all of clipmarks tags) at the very bottom of the list is the tag cloud link. That will bring up a whole page for your tags. The largest being the tag you use the most. the smallest would be the ones you have only used one time.
After seeing a discussion about where is the tag cloud, thought I'd clip this in case those who were in the discussion didn't go back to check the comments yet. - I hope this helps a little.
example for the link is
http://clipmarks.com/tag_cloud.aspx?u=123clipmarks replace "123clipmarks" with your user id and you can actually use the link to give your tag cloud address to your friends or contacts.