Tell us what you are trying to do?
Create a new user form. When I try to add fields, nothing is populating. It's just showing me a black box. I also can't click to drop down the CSS or JS editors.
Based on what I can observe from your screenshot, this appears to be a plugin conflict. What I would recommend that you do is to temporarily disable all your non-toolset plugins and try again.
Once you've done this please let me know if the issue remains.
If it does please provide me with admin access to the site so that I can have a further look at this one for you.