First: I just upgraded the whole CS to 5.5 last week, including Flash builder 4.5.3 Premium. To make that possible I had to use the Adobe cleaner, which resulted in manually resetting prefs in several apps.
I imported Flex Projects which use the SDK 4.6, so I got the latest (18.104.22.16801B) from the adobe site today. In the trial version of Flash Ubilder 4.6, there's no problem, but in the FB4.5 Premium you can't use Design View.
Trying to uninstall FB4.5 and then installing 4.6 trial doesn't work because even then, the installer says it has found an older version. So that's not an option!
I just can' tbelieve that the official release CS5.5 just doesn't support the 4.6 SDK. It seems I'm not the only one with the problem, according to other posts, so Adobe: Please, solve this right away, because as a prof.user, this costs me and I do not appreciate that!