Stretched Out, a third-party developer has released a Sierra Wireless AirCard 555 driver for Mac OS X. It costs $29.99 and even supports use with Virtual PC (most third-party drivers prevent Virtual PC use).
But, here’s the catch: The 555 is for Verizon, not Sprint (Sprint has the 550). But they are the same card, so they may or may not work on Sprint. The driver, like the Merlin C201 driver for OS X requires you provision your card in a PC laptop before using it in a Mac.