Bug Problem assigning animations

I've hit a roadblock in my progress assigning animations. The videos I've watched all describe exactly what I try to do. Click to create a new blend tree, go into the blend tree, assign a new motion, but I can't assign animations. I've gone through the process a dozen times and it's just not letting me assign animations to the blend tree.
I want to have an animation sequence for crouched characters to idle, or crouch-walk forward. (Will be adding directional variations if I can get this to work). However, when I click on the animation I want to assign to the motion the selection window closes and the animation "motion" still says "None (motion)" click the little circle, go through the list of animations in the window that pops up and double click it, and the window closes again still with "None (motion)". Can't drag and drop an animation into the slot, can't select it from the windowed list pop-up. What should I do?

 

Still having an issue with this, so, if anybody has any suggestions?