Shared computer only shows up under "All..." where no Login is possible

In our home network my wife's MacBook Air (2012, MacOS Lion) cannot see my Mac Pro. Actually it does, sort of, under "Shared" in the Finder, it sees it under "All..." where my MacPro (2008, Mtn Lion) then shows up, but she there is no way to log in like it would be possible if it showed up directly under "Shared".

How can my MacPro be forced to show up directly in the Shared column?

Our network is wired, restarting router and restarting machines does not help. Ping via terminal works.

Any ideas?