Well, its been a long two or three weeks of Construct 2 usage, and I'd like to save new users hours and hours of time that I wasted figuring things out that should be made very obvious in the tutorials.
1. If you're using arrays and its returning 0 and you're positive you set up the array size correctly and you're positive you used the "set value" technique correctly,you have to reset the size in the trigger/action section before you add any new values. Seriously, even if you run your program with an array defaulted at 10, 10, 10 and try to add "8,1,1" it will not work without a resize "10,10,10" before it. I'm not kidding...
2. BUTTONS DON'T SHOW UP IN MOBILE APPLICATIONS!?!? you have to use your own custom sprites, as far as I can tell. Don't even bother with buttons, they're hardly customizable anyway.
3. If you're adding a key to a dictionary, that's totally different than adding an instance variable. To my knowledge keys serve no purpose.
4. CocoonJS errors can be accessed by clicking on the FPS button in the top left corner. YOU SHOULD BE EXPORTING TO COCOONJS EVERYDAY because it likes to crash when loading certain images, and playing guess and check after 2 weeks is a huge pain and often project-crippling experience.