I don't think your function is doing what you think it's doing. What exactly are you trying to accomplish?

check to see if a given int is present in an array of ints. it should return true if the int is NOT in the array. 

" Processing gives me an error saying that the return type must be boolean"
Not really. You should quote exactly error messages you have...
Actually, there are cases where your function doesn't return anything. That's what the compiler complains about. It doesn't know what to return in this case.

The case where it doesn't work: ary has a length of zero.

And indeed, without code, you exit on the first loop!