You can use Samba, set up samba on your Ubuntu box, and you should be able to access the samba share from your Mac.
Since OSX is based on Unix, you can also use NFS. Set up NFS on your ubuntu box, and download NFS Manager for your Mac. The NFS shares will show up under "network" in the finder.
Sorry I can't be of more assistance right now, I am away from my Linux box so I can't check the configuration files. Hopefully I have pointed you in the right direction though.