I have Snow Leopard 10.6.7 on MacBook Pro 13" running in 64bit mode. Fax works just fine. I have an apple usb modem installed on my Mac Mini Core Duo running Snow Leopard 10.6.7 32bit mode, and I can send Faxes from the MacBook via the modem on the MacMini. Everything is working fine.

Don't know about Faxstf, as I just use the built in software.

Just to be clear, the newest 2011 MBPs boot by default into the 64-bit kernel, unlike older MBPs. Everything from the Core 2 Duo on runs Snow Leopard or later with a 64-bit OS.

There's a good chance the FaxSTF software uses a kernel extension that's 32-bit only. Try rebooting while holding down the "3" and "2" key.

If it works that way, nag Smith Micro for an update...