Did you set at least a arrival procedure? Was the cruise altitude set? Did you complete the FMC preflight?
Procedures are not automatically loaded via Simbrief but you will need restrictions in your flight plan for it to come up with a vertical path.
The G1000 NXI doesn't support user waypoints yet afaik.
As it was now implemented in other Avionics like the G3000 etc. I believe it will, at some point, also come to the G1000.