Help please.

A friend is connected to the net with Plusweb in Freo. He took their broadband setup despite being told they cannot help Mac users (what the!). They have supplied him with a 'Billion' USB modem and I have loaded the software for the modem, but cannot see how to change the mode of connection from Ethernet to USB connection. Anyone direct me on doing this or are Macs not USB compatible?

350mhz slot loader iMac plenty of memory.

TIA

Kev