I don't understand why you would need access to the modem, you port forward on the router, not the modem.

You need to do it for both the modem and the router. That's what I needed to do with my last ISP. Reference: PortForward.com - Free Help Setting up Your Router or Firewall

Hmmm..well, Portforward don't carry a guide for your modem model but the guide they have for another Arris model quotes an IP of 192.168.62.1 for the config pages.

I already tried that but it didn't work.

The modem is simply for receiving the broadband connection from your ISP. This should be connected to the WAN port of your router. You set up port forwarding on the router not the modem. If it's any different then I'm afraid I cannot help you. Many suppliers lock down their modems to stop users un-capping them - if you need access to the modem then speak to your ISP.

I guess I'll have to do just that. But I still don't know how the guy that came to setup the connect was able to get to the interface so easily but I can not. That's the thing that's bugging me.

Yea, it would bug me too... Good luck.