I'm having a similar problem as this person.
I'm trying to work with an old project that uses 2.0 framework in visual studio 2010. However it won't recognize that the System.web.extensions dll is part of that framework when it filters to 2.0 framework on the add reference dialog. And no, my framework is just set to ".NET Framework 2.0" as there is no 2.0-client.
Any help would be appreciated.
Right now I just manually went into the csproj file with notepad and added in the reference. This seems to work. Is there a way that I can do this with the other projects I'm working on, or will I just need to manually add it in the csproj files for each one?