You can add galleries to your directory two different ways:
1. Initial Gallery Creation
When first creating a gallery, the screen that prompts you to add your gallery title will also have a checkbox lower on the screen for you to add the gallery to your directory.
Note: you will also need to Publish your gallery for it to be viewable in your gallery directory
2. Directory Tab
The second way for you to add galleries to your directory is via the Directory tab located inside the menu option for Galleries.
If you scroll down the page, you will notice two columns. The left column entitled, "Directory Galleries," and the right column, "Non-Directory Galleries."
The left column lists all the galleries you have in your Directory and the right column lists all the galleries that are not in your Directory. You can easily Add or Remove the galleries in your Directory and publish or unpublish your galleries right there.