Port forwarding is a procedure specific to a NAT firewall/router, so technically - you can't "port forward" a modem - unless it's a modem/router. Since the Telus has the provision for port forwarding, we have to assume it's a modem router.
Next multiple routers in a single network are a major source of grief when it comes to port forwarding - you need to get rid of one - since the Airport Express cannot function as a modem, the Telus will have to be the one to go - ask the ISP how to put it into "bridged mode", which is a mode that removes the router functionality.
Once you have the telus in bridged mode, you can then configure port forwarding on the airport express.