Mine was preset for XOHM and would not see Clear until I flashed it with the most up-to-date N810 build. After that, it was fine and I could connect to clear, but the built-in MicroB browser didn't always work correctly with Clear's authorization website.

When you connect from a device with an unrecognized MAC, Clear dumps you to a landing page where you can either "log into your account" or buy new service. Choosing the "log into your account" option allows you to log in and then authorize the MAC address of the device you're connecting with. If you only have a one-device plan, doing this will de-authorize any other devices you have (like laptops or USB sticks).

The other option of course allows you to purchase service right from the device. I know they used to offer day passes, but I can't find anything on the website.