Its in the following location in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\EminentWare\Data Grid Service
Look at the ConnectionString Entry:
Data Source=DBServer\Instance;Initial Catalog=EminentWare;Integrated Security=True
"DBServer\Instance" should be the database and instance where Patch Manager is currently stored.
Here is a document that discusses it further:
Just be aware you cannot use an existing database with new install.. The database is keyed to the install.