I purchased the DIY Flame Blue Hands + Watchmaking Kit | 38.5mm Mosel Series Edition and have followed the directions exactly to the point of the plastic movement holder ring on top of the case back. At that point the movement is supposed to be placed in it. Mine does not fit, it does not seem to be an issue of pressure as others have noted here. The little protrusions meant to keep the dial in place are not in line at all with the moment. Has anybody else had this issue?
I had a problem with my pilot watch that was similar to what you described. Although I do believe my tabs lined up. When it came time to seat the movement in to the plastic holder, one side of the movement would not seat and stuck up. I was hesitant to put to much force on the movement to get to be flush. However, I carefully applied more pressure and it snapped into place. I was surprised how much pressure it took, but it eventually got to where it needed to be.
With regard to your tabs not lining up, it is possible you have the wrong retaining ring.