Make sure the back side of the tire is in the valley before proceeding. I use wedges of plastic to force the backside into the valley. This allows the "topside" to easily mount. You can see them on the right. At this point I would still try the tire out as long as the bead isn't destroyed.
Trust me, putting these 185/55 x 17s are a big pain and I swear I'll bring them in next time and let a pro do it.