IMac external USB modem

Could anyone help me with the procedure for connecting an external USB modem, to an IMac that has an internal modem. It seems as the software is not “accepted”. When the USB modem is connected it is found but the software that was installed is not. I’m using a Zoom faxmodem 56K. I have followed the directions that came with the modem. PS. the internal modem had a lightning strike. Pls. help this is driving me UP the wall. Thanks John