Did you read the manual?
1:- No, you don't even need to download or export from Simbrief as the 146 can get it from Simbrief itself
2:- You need to enable "GPS" flight, or R NAV. There is a nice little button for this:
[image: rnav.png]
3:- Enable LNAV [image: lnav.png] (lateral navigation)
To load the Simbrief flightplan I recommend this video:
https://www.youtube.com/watch?v=SgZGR0huR18
The same process as SB if you really like the MSFS flight plan, but the option below it.