Blend Tree parameter drop down is blank

I'm trying to create a new blend tree in the animator, and although I have 4 parameters, none of them will show up in the drop down list in the Blend Tree.
What am I doing wrong?

EDIT: Apparently only Floats can be used in Blend Tree. It doesn't seem to mention that in the documentation, so I assumed Ints could be used as well....

Now I have another question: Is there any way in which I can move around the Blending Tree's nodes? They are covering other nodes.

 

You can't move them around, but if you fill out any missing motion fields or sub-blend trees it should arrange itself so nothing is covered. Usually the overlap occurs when you've added, say, a bunch of motion fields but you haven't yet assigned animation clips to them. Once you assign the clips, they'll usually space themselves out properly.

 

Thanks, after filling in the empty motion fields, the nodes did properly rearrange themselves!