In order to allow for check numbers to be entered, as well as for admins to process registrations through credit card, you must enable the event option.
  1. Select Fundraising > TeamRaiser
  2. Edit the TeamRaiser event
  3. Click Select Event Options
  4. Under Related Actions, click Edit Advanced Options
  5. Under "Offline Gift Payment Recording by Administrators," mark the checkbox next to "Yes, allow administrators to input credit card information as well as cash and checks"