Visual Studio Schema Compare frustration

In Visual Studio Schema Compare, if you have a problem where the Export to Editor is grayed out, the Write Updates is unavailable or the Schema Update Script window is blank, it is in my experience one of two things.

1. One or both of your connections is to a SQL Server, and you don't have adequate permissions.  See here.
2. One or both of your connections is to a Database Solution, and you need to rebuild your database project(s).  If you can't rebuild your database because of errors... fix them, and rebuild the database project.  Make sure you have the sqlcmdvariables chosen for the compare and setup correctly between your database projects, if you have more than one.