irst off, take that code and indent it properly - so everyone can see easily what is going on. Then add some comments. Then replace your magic numbers with constant names, or the actual characters. Which of these is easier to read and understand:
if (ch == 42)
if (ch == '+')
if (ch == OPERATOR_ADD)
Then we can look at the problem a lot more easily.
But I'll give you a hint: Look at what is in your input string, and also look at what you push and pop on the stack. What characters are you getting from the file? What values would you be adding if your code worked? How many items are you putting on the stack?