Been a long time, but I don't think you can select a unit on the map.
You are going to have to look for the name of the unit in-game and then scroll through the units in the force editor to find it.
The best, I believe, is to note the name of the unit and it's force name, as all units are grouped under the force it belongs to. So, you look at the unit in game and note the name of the unit and the force name. For example, 1st Panzer Div (name of unit), 1st Panzer Corps (name of formation). Now go into the editor and look for the force it belongs to (1st Panzer Corps). Once you find it, the 1st Panzer Div should be right there.
Formations are the heart of the force editor. They can be difficult to manipulate and easy to screw up. For example, it is easy to copy a unit and rename it. But then that unit is not part of the force!
Hope that was of some help. The best, of course, is to read the manual if you plan on making any real changes.