Hi there! I think I can see what's going on there. Are you by any chance running your Mac at a fairly low screen resolution? ( 800 x 600 for example ). If so, you may find that the OK and Cancel buttons on the New File window will be clipped by the bottom of the screen because there's not enough physical room to display it.
If this is the case, you can still access these commands by pressing <enter> for OK or <esc> to cancel. If you're running a somewhat larger resolution but are running ArtRage in a window rather than fullscreen, you may want to try maximising it by hitting the little square next to the cross in the top menu bar inside the Window.
A new update we're working on at the moment is planned to change this behavior so that when people are running low screen resolutions we display less previews, thereby allowing this window to be shrunk down. You may also want to see on your Mac how it looks if you access system preferences, displays ( take a note of your current resolution so you can change it back if you don't like it ), and select a higher resolution. It sounds as if your monitor is large enough to easily take advantage of this resolution which will give you a little more screen real estate. If you prefer the previous look you can change the resolution to its original value.
Do feel free to get in touch if you're still stuck or I can be of further help. You can also email me directly at firstname.lastname@example.org