I installed the Upfront theme and then installed the Fixer

I installed the Upfront theme and then installed the Fixer theme and activated it. Everything seemed fine, but when I click Upfront or Customize is just brings me to the home page of my website. If I click the Upfront button again from there I get a popup message. I attached a screenshot of the popup message. Either Upfront isn't working, or I can't even figure out how to access the editing mode. (I'm a brand new user.)

This is for a client site, not mine, so I was trying to avoid installing the WPMU dashboard if possible.