What error message are you getting when you try to set up email in BIS? This will help determine the solution and end speculation.
The typical problem is the previous owner did not cancel their BIS account when they stopped using the BB. Hence, the IMEI and PIN are still registered. If this is the case, you need the previous owner to call his or her cell carrier and cancel their BIS. This will release the PIN and allow you to use it for RIM related services such as email.