I'm also using a Mac and when I want to log in using SSH I simply user the application Terminal (/Applications/Utilities) and typ ssh firstname.lastname@example.org. Switch username and server to match with your login details.
The you will have to type in your password on that user (same as for FTP).
Then you will be working in a new shell, but that shell will be running on the DreamHost server and the output will be sent to your screen. When you're done, type exit and your session will be dropped.