To add new media sources, navigate to Settings at the Account level.

In the media sources section, find the new media sources in the Available column, select the ones you'd like to add, and then click Move to the allowed section. Once sources have been added to the Allowed column, click Save at the bottom of the page.

Next, navigate to the ad group(s) where you'd like to enable the new source and click on the media sources tab.

Click on the + Media Sources button and add the new media sources by clicking on them in the dropdown menu one by one. Once finished, the newly added media sources will be enabled by default in this ad group.
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Keep in mind all ad groups that were created after you've added the media source(s) to the allowed section will have the media source(s) active by default.