How do I measure distance in ExpressPCB?

To make measurements you can use the mouse and reading its position on the status line.  Be sure to turn off the “Snap to grid”. You may find it helpful to select “View” and then “set origin with mouse”, so that you can make easier measurements.

You can also set/read the exact coordinates of any pad by double clicking on it.

Here is a quick video showing how to do this.