AT&T Phone with Fido SIM Card, can't access the internet...

First you need to have the phone unlocked, which I assume you already have, otherwise it would give you an error with the Fido SIM.

Second, you need to enter the Fido Access Point (APN), MMS and Internet Profile information into the settings.  Fido may be able to send the APN and MMS configuration as a push message.  The Internet Profile  you'll have to manually set up.  This post describes how to create a "no Internet" profile, you'd be doing the same, except entering the Fido information instead of the bogus info.