In SSDT 2013, I can't reference another project in the same solution. That project manages another database on the same server. It lets me pick the project, setup the database name and variable, but the OK button is disabled.
If I pick the option that it is in the same server, the OK button enables (but doesn't help, as it is in another database on the same server).
What is causing this? Am I missing some configuration on one of the projects to allow referencing?
In the "Add Database Reference" dialog box, try deleting the contents of the "Database variable" text box. That should enable the OK button.