Go to extensions -> edit tileset.
Add all the tiles you want by clicking them. You can have up to 16 tiles total in your map.
Once you are done choosing the tiles, click "OK" (important: it will save the map for you!) and then close your map. Reopen your map and the tiles should be there.
The important thing is to close the map after you've added the tiles via edit tileset. If you save it, then it won't save those tileset settings so it won't appear when you reopen it. Just click "OK", close the map, and then reopen.