Confused about wiring my Recent Purchase

You can most certainly bond the neutral and ground in this sub panel because it is in a separate building and you only have 3 wires. This was perfectly legal for many years even for new construction. Code changed in 2005 or 2008 I think to require 4 wires to an outbuilding. Since i assume you're not installing a new underground 4 wire feeder to the garage your neutral and ground MUST be bonded in the new panel. If not then the ground at the garage won't clear a fault because the earth won't provide a low enough resistance path back to the bond at the main service at the house. Code prohibits using the earth as a current path anyway. This is what your atempting if you don't bond in the garage.
Now, you mentioned metal conduit. If you have continuous metal raceway between the buildings it is your ground conductor. That means you really have a 4 wire feed and so should not bond in the garage sub panel.