PTC has rebuilt their licensing system since the release of Arbortext 6.0 products. Consequently, if there is any glitch in your maintenance records with PTC, the various options won't be there.
In addition to that, the method in which you navigate through the License Manager has changed dramatically. You now return your license, get your new server code and then go to the screen to get a license where it should have a list of the products you currently have a maintenance agreement intact on. There will be two listings for the Editor. Choose the one that has 5.3 listed in the drop-down. When you get to the next screen, you should see the option to select an Editor Fixed key. Any of the 5.x Fixed Editor listings should work for you, depending on when you bought the license.
The difference on the screen between Fixed and Floating (or Concurrent) is that the Fixed licenses don't have an option to fill in a field for the number of licenses you want since there is only ever 1 to be had on a Fixed license.
In answer to your question about Fixed/Floating, none of that has changed. They remain very distinctly, two different types of license keys. You still get your server code from the machine you're trying to license. You'll get a key immediately on the License Manager, as well as one will be e-mailed to you.
Hope this helps,