I see the same issue. FP is most definitely installed and I am running 2008 x64 and the panel says the extensionas are NOT installed. It might be as simple as the path that is checked for the install. On Windows x64, the path is:
C:\Program Files (x86)\Common Files\microsoft shared\Web Server Extensions\50\bin
By default so maybe the DotNetPanel is not checking that path? Just a thought.