Aliant's tech support has not been the same since they farmed it out to ICT. If you have any questions it's better to hit a forum and forget calling them.
The speedstream modems allow you to access the firmware and enter the PPPoE info right on the device. I normally bypass this completely (and forget about any installation disc or suggestions given by Aliant) and just enter the info straight into my router (in your case your Airport). The info you need to configure it as PPPoE is your username in the format as follows:
where xx represents your 2 character province code (In your case NB) and you'll also need your password. historically when your account is generated your password will be the first letter in the month your service was created and then your telephone number. For example, if your phone number was 902-454-8500 and you connected your Aliant Internet Service for the first time today, your password would be a9024548500.
Bottom line is this: Of course Aliant is telling you that you need their wireless modem, because they charge an extra monthly recurring charge for it