Hello,
We had Matrix for DD (now 6) and DS (nearly 4) and as Diva said it was supposed to be very safe in the lie-flat position in one Which report and then after we had used it for both is suddenly not safe!! Not sure what to make of that but we had no problems at all (even used it for DD to sleep in overnight a few times when she was tiny)
You should have seatbelt clips with it, they were red and sort of triangular, or were when we had ours. If you have them its best to leave them on the seatbelt once you have them in the right place.
Whereabouts are you and where did you get the Matrix? If you are Herts/Beds you can come round here and see if DH can remember how to fix it in the car - he has done it for a few people over the years tho may be a bit rusty! If not then most places selling them will show you how to fit it tho some are better than others. If in doubt turn up at a shop, ask someone and if they aren't helpful bring all your hormones into play, weep copiously and I'm sure they will decide to help (I don't usually advise this strategy but its a wonderful way of a pg woman getting things sorted)
Good luck