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There are a couple of possibilities. First, it might be opening in the docking area. So if you are expecting that big ol' dialog in the middle of the window, instead, if might be contained in the docking area. You can drag it back out, if you like. Grab the bar which has the tiny triangle and x buttons, and drag away.
Second, if you're using a dual or multiple monitor setup, or you recently were using it, the dialog might be opening off the screen. There are techniques to bring it back to the screen, but I don't know the procedure for Macs. Maybe someone else knows, or maybe you could try searching?
I see you report using Inkscape 0.91. For me, using 0.92.3, Document Properties, undocked, consistently opens almost completely offscreen. Only the left edge of the dialog barely shows on the right side of the screen, for me. But I didn't have that experience with 0.91.