Ok for those out there that are still way behind on moving to Team Foundation Server and still use SourceSafe, you will find adding solutions to it in VS.NET 2010 is not enabled by default. To switch source control providers back to SourceSafe do the following: Goto 'Tools' -> 'Options' and select "Show all settings". Expand the "Source Control" node and then select "Plug-in Selection". On the right hand side change the value in the "Current source control plug-in:" to "Microsoft Visual SourceSafe" (pictured below).
Then sometime soon consider moving to Team Foundation Server, which has a "Basic" version that allows a route into a new provider for those intimidated by switching away from SourceSafe.