To add units, you must have a prototype for the one(s) you want, then press SHIFT+F1 (IIRC), which will present you with a list of available units to choose from. Existing units can be modified by right-click on them (morale, owner, home city, etc.), except for needlejets.
Scroll lock switches between Paint Mode (for map editing) and Play Mode. To alter terrain, used ctrl+left click for "add/plus" effect, and ctrl+right (eg. raise terrain, place road, etc) click for "remove/minus" effect (lower terrain, clear improvements, etc)
I am not aware of any on-line documentation for this sorry excuse for an editor, but you'll find that a lot of things can be done with the right-click menu, the Map menu and the Scenario (duh) menu. I learned what I know through trial & error.
If you have specific question I'll be glad to answer them as well as I can, but I don't have time to write a user guide
Aredhran