1. Check under the 'View' tab in RME. Make sure creature and spawns are visible.
2. Make sure you place spawns with the creatures. Or else the creature won't save.
3. When you import a monster. Save your map(CRTL+S) and then hit F5 to reload. This will reload RME's Palette selection, such as updating the creature palette with your imported creature.
4. Make sure you have saved your -spawns.xml and -house.xml files in the same directory as the map, along with the same name.