To put those units in the game you need to change 2 files and create 1.
The 2 files to update are Civ5ArtDefines_Units.xml and Civ5ArtDefines_UnitMembers.xml.
In those files you will tell the game what are the skins for your units.
Just keep in mind that you can have those 2 files only one time.
So if you are using a mod which has already those 2 files you will have to edit them.
The third file will be a unit.xml or myunits.xml or whatever name you want to give it.
In this file you will put every info needed to create a unit : unitclass, unitinfo, freepromotions, flavors, IA,... .
For more details you can see the tuto on creating units and/or use Gedemon's "R.E.D Modpack" (
http://forums.civfanatics.com/showthread.php?t=383375) or my mod "Ethnic units" (
http://forums.civfanatics.com/showthread.php?t=409124).
Those 2 mods are using those units and you can easily find in them how we did it.