No worries! Happy to help.
The weight class die are used to roll on the Hazard table when a player triggers one (the !!! symbol) and as part of applying damage in a collision. So for one of the medium weight vehicles you'd roll 2d6.
It's not a perfect system, but it was intended to weight the average results of the table to different vehicle weights and give a slightly variation in damage depending on what you're slamming into someone with.