The installer tool is not needed if you put the rear main in correctly as Cummins intends it, reason why there is no installer tool. You need to pull the rear seal retainer off the engine and just install the seal the correct depth then use a new gasket on the block surface and RTV on the oil pan rail. Remove all old gasket material and clean thoroughly, let the RTV set a little before install seal and retainer while letting the seal center itself on the crank. A fair amount of comebacks after rear seal replacement is due to the seal being off center hence the better way to install.
FYI, if you have many miles on the engine you will probably need a speedy sleeve and its gasket.