It seems that Unity doesn’t yet support VS11, even though VS11 is backward compatible. So basically simply calling the VS11 executable just like the VS10 (2010) should be enough to work well.
I tried giving the path the dev_env.exe of VS11 but it launched instead the default editor, which is a bit odd.
It seems to work now. Unity 3.5.3f3, VS 2012 RC v11.0.50522.1 RCREL
Unity 3.5.x works perfectly fine with the VC 2012 release candidate.
You simply set the dev_env.exe as you did and it appears in your list of editors.
If you’re looking for an integrated solution, you can checkout UnityVS.
UnityVS is a third-party plugin for Visual Studio 2010 and 2012 that let you write and debug your Unity games easily from VS.
Disclaimer: I founded SyntaxTree, the company that does UnityVS.