It's been about ten years since I worked on one, but I remember it as being quite mysterious, until I figured it out and then it seemed head-slappingly simple.
I think you need to pull the power supply out before you can get that shelf out easily. Treat it like a Chinese Puzzle and examine what's holding it together carefully. There is a simple, yet non-obvious way of getting that shelf out. But as I said, try removing the power supply if it will come out, first.
Have you tried doing a Google search on PM7100 disassembly instructions?