How do I create a gallery after installing MoreGallery?

MoreGallery installs itself as a custom Resource, so to create your first gallery you will need to create a new Gallery Resource. To do you can right click anywhere in the tree, choose Create in the context menu, and then choose Create a new Gallery Here. 

After saving the resource a first time you can start adding images. 

Also have a look at the MoreGallery User Guide for more information on managing galleries with MoreGallery.