You just need to take your sim card and place it into the Charge and that's it.
I work at VZW, and was helping a customer with his Pantech 4G modem. While I had that modem in my possession after the call, I curiously swapped sim cards from my Thunderbolt to the Pantech and vice versa.
After a minute max, I logged onto my online Verizon Wireless account and saw my current device was that USB modem and when I plugged the modem to a laptop and launched VZ Access Manager, it showed my phone number. So the activation is all done automatically over the air, nothing manual is needed. Very slick and nice IMO.
Also, you get unlimited data taking a sim card from your phone to a USB modem or mifi, as long as you stay on your plan as you did on the phone, even if your minutes go to waste, at least your data is unlimited (or at least the cap is larger than the highest cap available for USB, which is $80/10gb currently).