I’ve followed the instructions to attempt to profile my ASP.NET solution using Visual Studio 2012 Profiler: http://msdn.microsoft.com/en-us/library/ms182384.aspx
However, when I open the right-click menu, the “Attach/Detach” menu item is disabled.
What do I need to do to enable this feature?

Apparently you cannot attach to VS Profiler to a process when using instrumentation as the binary has to be modified by the profiler and then started.
However, you can use the command line:
http://social.msdn.microsoft.com/forums/en-US/vstsprofiler/thread/f2f291da-8e83-4fa4-b4cb-a184965bfe16