Press the knob to get the cursor to display. Outer knob for Left-Right & inner knob for Up-Down (or visa versa. Buggered if I ever remember until each time I do it). The top of the screen will change to display your distance from the tip of the cursor. Move the cursor to whatever point you're interested in and you'll have a pseudo-DME to that point until you press the knob again to deactivate the function. It'll even highlight points on the map eg navaids, waypoints, airports or whatever is shown as the pointer touches each one so you'll know if the distance is in fact to the feature you want.
I use it all the time doing approaches & arrivals. It's handy to have a ready made distance to say, the threshold or MAPt while still several route segments away.
You can zoom in & out without affecting things other than feature resolution so handy if the feature you want is buried amongst other map features. You can also use the Direct To button if you should want to fly direct to the cursor position. The cursor position can also be turned into a user waypoint by pressing Enter.