Hope this helps:
S&W M&P22 Pistol - Shooting - Disassembly - YouTube
By the way, I rec'd my M&P 22 last week and I noticed the lever was quite resistant to pulling out the required distance for field stripping. I started to wiggle it along with slight pulling pressure
once I noticed the lever flexing. I was afraid of doing just what happend to you. I suspose it will loosen up some with use.