Hi and welcome to the forum!!
For the animator actions I find that if you type in the exact (case sensitive) name of the parameter you are trying to get/set, instead of chosing a variable from the list, then playmaker picks it up easily and does what you need.
So whatever your bool is called in animator just type that into the parameter box. As long as youre targeting the right gameobject (which holds that animator with that bool), you will get/set that bool.
If you did want to actually use a variable for one of the many reasons that it is useful in, I believe you will need to use the get/set property and save the animator's bool into a playmaker bool. Then you can just chose that bool (but ive never done this, just think it should work theoretically). If you must do it this way to achieve what you want and dont get what Im saying, let me know and Ill break it down more.
Anyway for that part here is the get/set property tutorial:
But as I was saying initially, if you just want to control the animator in playmaker, typing in the name of the parameter you want to influence is more than enough in quite a lot of cases. (So just type 'Walking' where the action asks for a parameter).
Good luck with it, hope this helps.
