When you create a gallery with WordPress, you should see a checkbox saying ‘Tiled Gallery’. Tick this box to use WP Tiles for that gallery.
If you have other gallery plugins installed, it may not work (for example, Jetpack Tiled galleries are incompatible). If you don’t see the checkbox, please let me know – if you can with a list of installed plugins. I’d be very interested to learn about incompatibilities.