What is it and is it needed?
I've never heard of a rear main seal installer before.

http://cgi.ebay.com/ebaymotors/Kent-Moore-J-39084-GM-6-5-Diesel-Rear-Main-Seal-Tool_W0QQitemZ200190060239QQihZ010QQcategoryZ43998 QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Its like a bolt on press. After oil it up you put the seal in place or on the tool I can't remember then screw the tool onto the crankshaft (with the 2 thumb screws). Then turn the big "T" handle and it presses the seal in squarely and to the proper depth.

I borrowed one from a local dealer. I used it and it was real easy and I know it was a good install. Kind of tries to make the install dummy proof. Yep I wanted the protection. Note its almost impossible to make something dummy proof. Dummies are not smart they are just ingenious at finding ways to screw something up (thats me). If you have done a seal or 3 I think most very gently tap it into place with a small hammer as you work around it. Like any other seal don't pinch it or deform it etc and you are probably ok.

Thanks