I like games that are about transformations!
So I took a look at your cap.
I completely changed over the events so it works now with buttons like I assume you wanted to.
For the buttons I just used sprites on a non-scrolling layer with a PV that holds the name of the animal you will be transformed to when you click it. Then a function of the same name will be called. I don't know if you were aware of the function object yet, it's a very useful thingie!
I put all the creatures (Man and Ferret) in one group called Animals, makes things easier. Also you didn't seem to be aware of the Invisible on start setting, which you can just tick for stuff like the player hitbox for example.
Oh I also added the Mouse/Keyboard object which you need for many things. In case you didn't notice yet, you need to add this one manually.
Well just take a look. I hope this will help you out a bit.
EDIT: I almost forgot: download changed cap