try using simroutes.com, you can then download and save the route to the fs/pmdg/flightplans directory in your computer making sure on the website that you select pmdg as the download option.
So if you have just downloaded and saved the file into FS/PMDG/FLIGTPLANS and for example the route is EGLL to EHAM the route filename is probably EGLLEHAM so you then type EGLLEHAM into the CO ROUTE on the FMC when in flight simulator.  I hope it makes sense. Also you'll have to enter all SID and STAR info yourself on the DEP/ARR page of the fmc