Typically I search for a system in MyAutosupport and then when I've got it in front of me, I click on the Upgrade Advisor link and it knows that I want it for the system I was just looking at. The new version of UA requires that I search for the system again, this information should be populated from the system I was just looking at if I used the link from the page displaying the MyAutosup information.
I (as well as a couple of other people) in my office as seeing this same issue. When we attempt to cut and paste the serial number of the system into the search screen that comes up and run the search, it just hangs there on the screen saying "Loading" and never returns anything. See attached screenshot