Reinforcements can only be applied to scenarios (and scenarios in campaigns).
1) Purchase your entire force in the unit selection part of the editor.
2) Select the unit or formation that you want to come on as a reinforcement and while it is selected hit the numbers 1, 2, 3.... 8 keys. You can assign more units to each number. There should be an R1, R2, R3 against the unit purchase if you have done this successfully. These are Reinforcement Groups and will arrive based on the rules you set up from the next step.
3) From the left hand menu select "Reinforcement". Then select the number from below this. Each number will correspond with the numbers assigned in instruction 2 above.
4) Assign a radio msg in the text box that will appear for the player to alert them that reinforcements have arrived. For example "2nd Platoon on station"
5) Assign a time the unit will arrive. Note this value is based on the number of minutes since the start of the scenario. If you set this too high the reinforcements will never arrive.
6) If you want there to be some randomness make the selection from here as well.