There are two different ways the experience is converted into levels, etc at the moment.
The normal way aka
Default is the default way the experience part is calculated. When MyPet.LevelSystem.CalculationMode is set to
default all pets will level up like normal players would level up like in Minecraft Pre-Snapshot 12w23a.
This is the advanced way where you can create your own level behavior. When MyPet.LevelSystem.CalculationMode is set to
exp.js in the MyPet-pluginfolder to calculate everything. In order to use this feature the plugin needs the rhino.jar inside the MyPet folder (since MyPet 1.2.3).