Way #1: Play a game, gather rescources, construct a military building, create a unit.
Way #2: Go to the place object tool in the scenario editor and put a unit on the map
Way #3: Create a trigger: make a condition (like a timer for example), then create an effect -> place objectWay #4: You have to edit the game's core files to add a new unit. It can be done but it's a very long process with lots of things you have to do, such as, create a model/texture for you unit, work it into the XML techtree file (if you want to add it to the game), work it into the XML protounits file and give it it's stats and settings, then work it into the language.dll if you want help stats displayed for it etc etc etc.