If you can't even "open" the databases you pretty much cannot compare them. There are tools that work on dumps but they need the dumps to be in the expected format, and the easiest way to do that is load the server and do a dump.
You must first be able to start a database server for each database. The only way to read a PostgreSQL database is via a PostgreSQL database server.
It isn't clear what form the databases are in, you didn't specify that.
If they're dumps, just install a new enough PostgreSQL for the dumps and restore them to the new install. Use a separate machine or just install the new Pg alongside the old one. You didn't mention your platform, OS/version, etc, so I can't help further there.
If they're copies of the PostgreSQL data directory its self and were taken correctly from a stopped server, you must get the same version (not newer, not older) on the same OS and architecture and start a server from the database. So if the database is from (say) 32-bit Windows PostgreSQL 9.2, you need 32-bit PostgreSQL 9.2 on Windows to load the database, nothing else will do. Start the server with
Once you can connect to both servers you're in the position of being able to use existing tools to compare the two DBs. When it comes to mapping out DB structure I find SchemaSpy quite useful. For comparison consider apgdiff; this will work on dumps, but requires dumps to be in the right format.