The trick i've found with Mavic rims is to use a tyre lever to bead the tyre into the little groove at the inside base ot the side wall.
Pretend you're going to remove said tyre but only pull it to the inside of the channel.
Do that on both sides and you should be hunky dory
Pretend you're going to remove said tyre but only pull it to the inside of the channel.
Do that on both sides and you should be hunky dory