Once you have a working version of Unity, the MMORPG kit and your choosing Behavior Tree system (Such as Opsive Behavior Designer) and you have the add-on folder in your project you will need to tell the add-on which version of behavior tree you are using. 

This uses an appropriate #IFDEF variable in the project.

In Unity, go to Edit | Project Settings

Select "Player" and in that tab look for the section "Scripting Define Symbols"

Add the variable as required for your targeted Behavior Tree. If there are other symbols there already, you can append more with a semicolon in-between.