MyPet.jarand a folder selection opens. Select the folder where your skilltrees are located and when you selected a folder a browser window will open. All changes made on this page will be saved to the skilltree files when you hit the Save-Button. Most parts of the SkilltreeCreator should be self-explaining.
Add Upgrade-button next to the skill selection. In the new popup you can choose between a fixed and a dynamic level-rule. Fixed rules will apply the upgrade on fixed levels like 5 or 1,3,5 and 7. Dynamic rules can be used if you want to apply the upgrade over a certain range of levels like every 2 level (2,4,6,8,10,... or 1,3,5,7,9,...) for example.