You can configure a Donation Form to allow Offline donations to be entered for it by following these steps:
  1. Go to "Fundraising" > "Donations Management"
  2. Click on the "All Donation Forms" tab
  3. Find and "Edit" the desired form
  4. On step 1 "Identify Donation Form" look for an option titled "Interaction Mode" and choose either "offline-only" or "online/offline" from the drop down. 
  5. Click "Next" or "Save" to save the selected option