Try replacing MiniStack with stackPtr. You don't want to index the class (that is meaningless in this situation), you want to index the array pointed to by stackPtr.
Also, line 9 should be stackPtr = newchar[MINI_STACK_SIZE];. You are currently allocating an array of ints that is never being used.
That worked, thanks! May I ask why it's meaningless to index the class though? Why can't I access a point in the stack array like I would a regular array?