When I press Next after selecting a trial:
A problem was encountered while trying to load the trial period for Adobe Flash Builder.
Yes, I have installed it to a case-insensitive volume (even though I think that you guys should fix that. Not pretty )
Yes, I am running it as an administrator.
Yes, I have tried running the install.support/builder-common-mul/Setup.app
Running Mac OS X 10.6.1
Sounds as if you've run into this known issue on Snow Leopard:
FB-22869 - Licensing error prevents Flash Builder from launching on OSX 10.6.1 (Snow Leopard).
After installing on 10.6.1, run the mini-installer found in your Flash Builder installation folder:
Let us know whether or not that resolves the problem,