No, you're not missing anything. There is no GUI tool in the WHS 2011 Dashboard to manually manage server backups. That's what's missing! You're going to have to RDP into the server desktop and use the Server Manager, or, more likely, the
Command Shell to issue commands. Wonderful, isn't it?
As an example of the commands to delete the oldest server backups, take a look at Jon's comment (dated 14th Jan 2012) to
this blog post.