The salesman at Verizon told me to come back with my laptop so they can install the software and tether it to my Treo (700P). But for 15 bucks a month, it's not feasible for me.
Accoring to Verizon's site, if you run the setup wizard on the VZAccess Manager CD (if it came with your device, I have to see if it came with mine) you should be all set once it's done (and you reboot).
Here's the PDF of the User Guide.
As long as you have the BroadbandAccess Plan once the install is done, you'll choose
Detect WWAN Device when prompted and select
USB Port. Connect the Treo to the PC, enter your phone number, click
Connect, and you're all set.
Good luck!