If it anything like HQ then 1) Very carefully 2) Very gently 3) Roll it down from the top I would imagine. Could try bumping it with your hand to get it to clip on but may hurt, or you could try a rubber mallet being aware of 1) and 2). If the car has been painted you may have trouble with the extra paint thickness.
Place the top of the trim on the top of the gutter and with your hand, bump it downwards......This is how I did it with my VC, when it was painted