Yee,
From your comments, I assume that if you create an ER diagram on machine A and create another one on machine B, then both diagrams work as expected and your problem is trying to get the diagram created on A working on B, is that right?
What database are you connecting to?
The factors that are important for the ER diagram to be able to connect and refresh itself are the profile name, the catalog and / or the schema. If you open the mer file in a text editor, you should notice something like
<Connection session="Oracle-Local" database="HR" />
close to the top of the file; here “session” is the name of the Profile in the DB Browser view and “database” is the name of your schema. The attribute values for these attributes should be correct for both the machines.
I hope this helps,
Brian.