Hello. I'm in a similar situation. I'm new to Gramps but familiar with Mac OS X.
Preferences. When Gramps is in focus, see the top level menus Gramps, Family Trees, Edit, at the top of the screen? Click Gramps. Preferences is under there. Virtually all OS X applications have a primary menu that is labeled with the name of the application. Virtually all of them will have their Preferences placed under there and also accessible by command+' key combination.
The add-ons. They appear to be under the Tools menu. Like I said though, I'm new to Gramps.