First of all, download Minetweaker RecipeMaker, this has helped me a TON. Now, while your in game, make sure cheats are on, and type /mtrm. A GUI should pop up, click the box marked shapeless, and then put in the GUI a slimeball and a sapling. It should automatically oredictionary for you. The click save and done. Then I would test it out.
Cheers,
