That´s a response that was reported by owners of stick I and II here in Germany too. No longer crashing but not finding the modem either. Seems that the package for now just includes drivers for a few T-Mobile USB sticks.
That probably also means you can´t find it under "system settings" and then "network" on your Mac?
From what I´ve read here:
http://seekbroadband.com/hardware/3G-USB-Modems-and-Dongles/T-Mobile-Option-Icon -431-USB-Modem/
it might identify itself as an "Option Icon 431" modem/mobile? If that is your stick in the picture?
(Just as my stick is identified as a "Huawei mobile".)
If it´s there in your "Network settings" configuring the stick manually might be possible.
If not...
Are you still using Snow Leopard or are you back to Leopard?
If you´re back to Leopard you might consider "using" the 2 Aral Balkan links. And then updating again. In his case the USB stick drivers seem to have survived. And only those are needed for identifying the stick. The problem seems to be mainly with the T-Mobile software which he wasn´t using.
Although he doesn´t mention the type of his USB stick. It´s entirely possible that some drivers might survive the update while others won´t. It could be a huge waste of time....
And I´m not sure if I should even mention the "Option" website.
http://www.option.com/en/support/software-download/usb-modems/icon431/
Because they don´t say if the software works with Snow Leopard. And even more important the T-Mobile USB stick 530 seems to be SIM-locked? In a worst case scenario the Option software could overwrite something the USB stick needs. Which would leave you with a useless USB stick.
Long post without much help, I´m afraid.
As I see it you have just 2 options.
1) If there is an "Option Icon 431" modem/mobile in your network/system settings you could try the Aral Balkan solution.
2) If not, then you have to stay with 10.5.8 for now and hope for an update soon.