I'm trying to apply a behavior and all of the extensions on my panel menu are grayed (greyed) out and inactive?
99.9% of the time this means that one of two things is wrong:
Some extensions require that you have one or more "Layers" on your page or that you have a style sheet. So if the extension is one that manipulates Layers, then add one or more Layers to your page. If the extension deals with CSS, then make sure you have a style sheet linked to your document or embedded in it, some may also require that you have a custom class defined in your style sheet..
Dreamweaver sometimes saves us from ourselves by restricting access to certain behaviors based on the browsers you have chosen to support. To check your settings: