Vertical monitor setup with Yosemite
Depending on your version of Mac O/S there should be controls in "Displays" that allow you to rotate the display. On the Snow Leopard mac I have at work there is a menu to the right that says "Rotation:" and a menu with Standard, 90, 180 and 270 degree selections available in it.

I seem to recall it looks a little different in newer versions of Mac O/S