For those having issues with the following mod tutorial due to some of the things not working, please follow my guide below. Just a simple move of a single file:

Official: Modding AI behavior

So for all those unable to use the behavior tool, i was able to get it working. There is an issue with file locations that I was able to piece together. When you subscribe to the tool, ensure that you have the behaviorTree folder in the tools folder as described above. If you don't right click the ME (Medievial Engineers) SDK in the tools section in your steam library (how you downloaded it to begin with) and navigate to betas. Opt into the latest beta (0.3.8 at the time of this writing) and wait to download.

Once you make it to this point everything should be easy. Ensure that the VRage (TM) editor is closed. Navigate to the download location of your ME SDK. From there Navigate to:
Tools->BehaviourTools->Plugins->BehaviourTools->Content. Copy BehaviorDescriptors.xml to your medievial engineers folder found in app data (%AppData%/MedievialEngineers/)

Once completed, you should be able to follow the tutorial above.

~Notes to the developer(s)~

Keen SWH. Please implement and enforce some form of location and coding standards to maintain consistency (behavor/behaviour spelled 2 different ways, one way in def file that needed to be moved described in this comment and the name of the plugin). I know this is a simple mistake and only about 1% of your customer base is affected by this. From one developer to another I know it is easy to overlook simple mistakes. From reading the code, this looks more like misscommunication and not carlessness, as this is indeed trying to read from the user data folder. However, none of the other plugins are reading from the core ME user data location (as it shouldn't, this is a seperate applicaiton). They are reading from the VRage entry in user data. Also, the application feels a bit dated. It is evident in the use of the ribbon and WinForms. Being a dot net team lead, (spending most of my time making, and reviewing WPF and web applications (which is what granted me insight into resolve this issue)), it pains me to see such a progressive company using superceeded technology. I may be able to assist if you are intrested. You are doing great work and don't loose hope on this little gem that is ME. With the proper polish and a positive community behind it, this could rival MineCraft (TM), as long as the modding proves to be sufficient to give server owners power to fill niches you shouldn't be wasting time on. Yea right now you can take mod ideas as this is still alpha, but keep in mind that MineCraft (TM) was just a cohesive collection of mechanics (hunger, health (means to de/replinish), mining, building (means to achieve) and an RNG buff (enchantment)). Everything else is a mod or was derived from a mod. Keep it simple, embrace your modders (they keep your game fresh with little work on your end), and test extensivly.
GLHF Michal Wrobel and the Keen SWH Team

TL;DR;
1) Ensure you have BehaviourTree plugin
2) Ensure that the VRage (TM) Editor has been closed
3) Copy BehaviorDescriptors.xml found in:
SteamApps\Common\MedievalEngineersModSDK\Tools\BehaviourTree\Plugins\BehaviorTree\Content\
to:
%AppData%\MedievalEngineers\
4) added a note to the developer that you probably wont understand fully.