There are a couple of ways you could add Creative Commons licensing information. The most straight forward would be to log in and navigate to the description you want to add the licensing information to. Start at the highest level of description to which the information applies. From the button bar at the bottom of the edit screen choose More > Add rights.
For Basis choose "License"
For License terms, add the terms of the CC license that applies, for example "Free to reuse, remix, and share with credit given to the rights holder."
Add start and end dates if appropriate.
Add a rights holder, if appropriate.
Add any other information you want, and click Save at the bottom of the screen.
You will now have rights information recorded on your description, with links to the license itself (if you have added the URL), and to the rights holder (where you can add more information as required.)
If you want the rights information to appear at all child levels of description, or only those with digital objects, go back to the button bar and choose More > Manage rights inheritance.
This is a passive approach - if your audience doesn't notice the rights info they may miss important instructions.
The second method would be to enable the copyright statement to pop up any time someone tries to download a master image. To do this, first go to Admin > Settings > Permissions and set Enable copyright statement to "Yes". Create the message about the license that you want to appear when someone clicks on a digital object. Save and return to your description.
This time in the edit screen on the rights section, choose "Copyright" as the rights basis, and farther down the edit screen set Restriction to "Conditional". Now when a user clicks on an image, before the image appears the user will see and have to accept the copyright (license) notice.
You can read more about setting rights here: