When the modem is in bridged mode it does nothing except pass all traffic through to the SME box (I get the same efect with my router if I use the DMZ option); no port forwarding required!
There is a port opening contrib around for SME5.6 but I don't know if it works on SME6.
With the port forwarding question, why not just reconfig an app to use the port you want rather than trying to forward?
HTH