reply to SuperNet9
Payment center (if it's really Comcast run cable store, not a third party payment center) can type something into the computer, maybe even change your account to use that modem.
BUT safest would be to pickup the modem, bring it home, install (call in to tech support to register the modem if you can't get out of the wall garden without installing the software, they can switch it manually without you installing any software) and then returning your old modem (if it was leased) later (get and keep the recepit!!!).
this gives you a path to step back to the old modem, should something go wrong during the install (reg. server down, new modem defective, etc.)
Yes it means another trip to the cable store later on, but better than no working internet until you can get a tech sent out, or go get a different modem or, get someone to undo what the cable store did, ???