There's no software support for Apple's USB modem (but they no longer sell it anyway); there are several third-party modems compatible with Lion. Here are a couple:

USRobotics 56K Modems: USR5637 56K* USB Faxmodem

Zoom Telephonics - V.92 56K USB Mini External Modem


dkmarsh—member, FineTunedMac Co-op Board of Directors