Well, addLevel is an older method that was used in v2.5.4 and prior versions of Aspose.Cells for Java. Please use addKey() method instead in the new versions of the product if you need more than 3 keys/levels for data sorting.
Well, the addKey method is there in the new/latest versions of the product, see the attached screen shot. I am using Aspose.Cells jars in the JDK1.5 folder.