What is the exact error message and on which line of your source code does it happen? All the details in error messages are important, and give you exact information where and what is wrong. Read the error message carefully.
"Cannot resolve symbol" means that you're using a name (of a variable, method, class etc.) that the compiler doesn't recognize.
Looking at your code, I see at first sight that there are errors on lines 9, 10 and 11 of StDemo. If you want to call the pop() method on your class Stack, you'll need to write that as my.pop(), not my.pop.
You ought to put the  before the name of the reference. The compiler might not complain, but it is better style.
Then you see you are creating an int but assigning it to a st. Surely that ought to be a new int?