Can't really make a video honestly but here's a step by step:
1. Generate flight like how you would any other Simbrief flight, just using the EFB profile
2. Take the passenger count and multiply at 162.7 lbs or 73.8 kg and select approximately that many passengers in weight in the passenger section under Flight Performances > Weight and Balance
3. Select approximately the amount of cargo and fuel
4. Click Load Aircraft
You can then use the values given in the EFB from the info button to get Zero Fuel Weight and fuel on board. Takeoff weight is ground weight minus 200kg by default.

Sorry for the late reply - only now noticed the comment.
If you're still interested in that, you go to the MCDU main menu page, select options, and then weight & balance, and load it in through there, though this isn't encouraged because your Center of Gravity will be wrong, at least from what I recall. They may have fixed it

today it worked.