design mode is forbidden in FB4.6 with ApacheSDK

There is a hack you can do to get it working (I've done this myself and it works perfectly so far) that involves editing an XML file in the 4.9.1 SDK to trick Flash Builder into thinking it is using Flex 4.6. You'll need to edit the 'flex-sdk-description.xml' Version tag and change it from 4.9.1 to 4.6 and that should take care of it. Location of the file is in the SDK folder in 4.9.1 folder, here is the path in Windows 7:

C:\Program Files (x86)\Adobe\Adobe Flash Builder 4.6\sdks\4.9.1