if you're using FSnav and PMDG 747/737 then get the plugin from the FSnav main page which allows you to export your flight plans from FSnav to PMDG.

just do up your plan on FSnav, click export under the plan button on FSnav and export them to PMDG 737. (no 747 available) if you're using the 747 then go to the FMC and rte page, type in the default name (minus the .rte) of the route that you exported (PANCRJAA001.rte) <---- example and hit COroute (company route) and off you go.