On a new Mac Mini with Yosemite, I cannot figure out how to add desktops to Spaces. As far as I can see, the preferences are the same on this new Mac Mini as on my MacBook Pro, but on the MBP, when I go to Mission Control, I see the desktop bar with an add button that appears when I move the mouse pointer to the top right of the screen.
On this Mac Mini, when I go into Mission Control, the windows open on my one desktop shrink and show on the screen, and the dock shows at the bottom, but no desktop bar at the top.
EDIT: I have added a whole screen (Cmd Shift 3) screen shot of what happens when I activate Mission Control. As you can see, the browser window goes much higher on the screen than it does on my MBP, where the desktop bar is. Also, no button appears when I move the pointer to the top right. One thing I did not notice before is that it does not show me all of the app windows I have open when activating Mission Control, like it does on my MBP. On that machine, all of the app windows I have open on a particular Space do spread out and show up in Mission Control.