If you are using the VT-102 tool (part of the Communications Toolbox) and MacTerminal 3.0, you can issue the keypad commands from the Keys menu. If you are using a Communications Toolbox-compliant application, you should be able to issue the commands from the menu the application provides for keypads.
If you are not using a Communications Toolbox-compliant application, you may need an application revision that works with the PowerBook. That is, the PowerBook keyboard has a new identifier* inside the Macintosh Operating System, and some applications may not map the mainframe key sequences from the keyboard.
You can use ResEdit to get the ASCII codes for the keypad, but more than likely, you want the key sequences that the mainframe requires. You could then create macros to issue them, or remap them to different keys if the application supports that.
*In particular, the PowerBook keyboards return selector identification codes of:
gestaltPwrBkADBKbd = 12; {PowerBook Keyboard}
gestaltPwrBkISOKbd = 13; {PowerBook Keyboard (ISO)}
for the Gestalt "kbd" selector. If your application uses Gestalt to determine the keyboard type, it may not check for this result (since it is new with the PowerBook) and may not do the correct mappings for the mainframe key sequences.