Have phone, have account, want to activate

A friend gave me a used but serviceable Verzion cell phone (after she upgraded to a new one). I already have a Verizon land line account, so all I want to do is activate the cell phone. Sounds easy enough?

I went to the Verizon web site and the next step after selecting a plan is to purchase a new phone--they're essentially free with a 2-year contract--but there doesn't seem to be an option to avoid the contract and activate an existing phone.

How can I do what I want?

- Rich