You can do both. Either set up a default license in your account profile that gets assigned to all your content. Or you can choose to edit a particular presentation and assign it a license that is different from your default license.

To set up a default Creative Commons license for all the content, go to the Edit Profile section and click on Manage Account. You will get the Choose defaults for your content option, select the appropriate settings for your content.

To set up a Creative Commons license for individual content, go to My Uploads and click on the Edit/Delete link. Scroll down to the bottom of the page, click on the Choose a license drop down menu and select the appropriate license.