This looks like your mac can't talk to the other system's port mapper. Either it's not running (not sure if you'd be able to locally mount the disk then), or a firewall on the server, client, or both blocks the portmapper's port (111).
Make sure that a portmapper is running on both ends. Then, as a quick test, shut down the firewall on the Linux system and try again. If that works, add the proper firewall conduit, and restart the FW -- don't run without.
Post back if that doesn't work for more specific tests.
Hope it helps,