so how do you assign a profile to a folder?With Profiles, there is always one metadata field selected as a "profile trigger". Often, this is a special field (like a custom metadata field called "ProfileTrigger"), whose values are defined in a custom table/view.
what are some good forums for webcenter?It feels like you don't like this one ;-)
Folders can be configured to enforce metadata rules on their content itemsI believe this refers to the Force Propagation setting mentioned below. Myself, I'd probably use a softer verb than "enforce".
Is it possible to set a default value on a field (the trigger field) on all content that is uploaded to a folder?Yes, it is. However, there are few limitations you should be aware of: