I have installed PMDG 737 and also the Navigraph Addon for that aircraft.
BUT, when I try to open Simlink the app says that Simlink is already running.
My questions are:
How do I access Simlink to create a flight plan?
How does Navigraph/Simlink integrate or show in the 737 FMC?
If so please see P117-118 of the PMDG_737_MSFS_Introduction.pdf in your MSFS PMDG documentation, commencing at Using Simbrief to create a flight plan and weather data.
I shall assume you installed the standard MSFS Sim from Marketplace: If not please advise.
In SimBrief Downloader PMDG Flightplan section you need to add a Download Directory where SimBrief places the rte files created when you generate an OFP. This will be something like: