Well, James, let's begin with identifying the input data points that are associated with the keypad.
Follow the wires from the keypad to the Input module in the controller chassis, and identify which Slot number the module occupies (the CPU is Slot 0, and numbers increase 1,2,3.. to the right).
Input module data in an SLC-500 is represented as follows:
I:x.y where x is the Slot number and y is the Bit number.
Poke around on the keyboard a little bit and figure out which bits are turned on by each keypress. You might even want to write them out in a truth table.
You're going to need to learn about the Masked Move (MVM) instruction as well as the FRD (Convert BCD to Integer) instruction. Look them both up in the Instruction Set Help section of the RSLogix 500 programming software.
As with most programs, the procedure is more important than the code. Write down, step by step, what you thing the SLC-500 needs to do with those keypresses, and converting those steps to ladder logic will be a lot easier than just sitting down and starting to enter instructions.