To be able to make these changes, your user will need to have permissions to customize your CRM (the out-of-the-box System Administrator security role contains the necessary permissions).
In Dynamics versions lower than 9, go to Settings > Customizations or If you are using V9 and above, you will first need to click the Settings wheel at the top of the page and select Advanced Settings.
Once located, both Versions will be able to click into Customizations.
Select Customize the System, then expand Entities > Campaign Response
Open the Fields section, then select the field responsecode and choose Edit. This will open a new pop-up of the field settings.
At the bottom left, all available responses will be listed. To add another option, simply click on the Plus sign. The field will automatically be called item, but you can edit the label on the right side. You may also add a description of the response code.
Save and close to update the field. Once you have saved, publish all your customizations for the options to become available for all users.