In most of the cases, Enabler should pick up your browser’s proxy settings. But if for some reason you need to specify them manually, here’s a couple of tips:

1. Use Settings button in the Salesforce toolbar to specify proxy settings.
2. Make sure the server name starts with http://
3. One of our users had problems with an authenticated proxy because his user name looked like ‘prefix\username’. He was able to solve this issue by removing the prefix part and just using ‘username’