The red dot is normal, it just means it's one you imported yourself and it's not RTP. Now for the rest of it.
You need to go into the database-> tilesets. Click the change maximum button, change the number to how many tilesets you will have total. Then click OK. Click the blank area, name your tileset, and choose the graphic. Ta da. Hope this helped. Don't forget to set the passive/priority.
If you need pictures. Click here.