Is your project an "AIR" project? See if the "Convert to Flex Desktop project' is enabled:
Right click the project, "Add/Change Project Type", Convert to Flex Desktop
It might be this happens and other errors if you forget to Run Flash Builder 4.5 Buritto and startup the other version FB4.
It uses the same project listings and that can be confusing since you're not expecting the old version to be aware of new Buritto (MobileApplications) projects you added (thinking) only to the BETA version installed elseware no doubt.
Well it might not be your problem but worthy of noting for other's I'm sure.
Otherwise you've double checked you're using SDK 4.5 Hero with it too! right?