Hi Egll,
If you have set up the export options correctly, fsbuild will export the built flightplan to both fs2004 and your PMDG 737 flight plan folders. To load the flightplan into FS2004 so you can use the default ATC just start FS2004 go to the flightplanner and use the load button. This will give you the list of saved flightplans, you then select the one you have just created and click ok. this will load it into fs2004. For example, if you built a route from EGLL to EGNT fsbuild will export it to FS2004 and call it EGLL-EGNT. Just select that route and you're good to go. The same applies to your 737. The route will be created and given the name EGLLEGNT. you just have to type that into the FMC and upshift it to the CO Route field on your FMC then activate it as normal. The route will then be programmed into the FMC.
One caveat when using FS2004 ATC I would exclude SIDS and STARS when buiding your fsbuild flight plan. I'm not to sure how the default atc handles them although you could always experiment

.I use Radar Contact 4 and it handles them fine.
Hope I've been some help and not confused you further
Steve