I've installed Extension Builder 1.5 on OSX.
I am working in a large organisation behind quite a paranoid firewall, so I'm getting a hang on creating a project; all I get is a "Contacting server" window that forces me to close Eclipse as it never goes away.
Is there really no provision for users behind firewalls or with limited/intermittent internet access?
Extension builder will dial home every few days - I think the blocking dialog is only shown if you haven't contacted the server for a week or more (or if it's the first attempt). If you use the machine outside of the firewall once a week then that should allow extension builder to connect and function normally. I'm surprised that the request is blocked by your firewall, as it is just an HTTP request - I'll investigate this further.
The dialog should also not be blocking - we haven't seen this behaviour before, and I would imagine the firewall and our timeout code aren't playing well together - I'll look into this too.
Note that extension builder only tries to authenticate if you are using extension builder functionality - you should be able to work on non-extension projects in eclipse and flash builder without being presented with that dialog.
Hope that helps,