My ctrl+ isn't working with Greenfoot specifically. Zooming out with ctrl- IS working, however... As a result I am now completely zoomed out and don't know how to zoom back in so that I can actually read the code again.
While in the editor, select 'Options>Preferences...' from the menubar; in the 'Editor' tab, change the font size to 14 or so and click the 'Ok' button.
From outside the editor, you can get to the 'Preferences' dialog box by 'Ctrl-,' (control-comma).