If you find that the units of your scale bar are in pixels instead of um, follow these instructions for an easy fix!
1. Click on the "Scalings" button. It has a picture of a ruler, and is right next to the scale bar button.
2. Make sure 40x Oil is highlighted with a checkmark next to it. If Pixels is highlighted, double click on the 40x oil line.
3. At the bottom of the popup screen, click "Apply to Selection"
4. You should now see your picture. You can double check that the conversion has been made by either drawing another scale bar and/or checking at the very righthand part of the rulers along the picture that it says "um" instead of pixels.