Considering that you have Cygwin and the Bison package already installed, you run it as following:  
bison -d calc.y  
gcc -o simple-calc calc.tab.c  
./simple-calc  
After doing this you can then "enter" any expression you want.  
After pressing the "ENTER" key you will get the result.  
For example you could do:  
1 + 2 <ENTER>  
This would give you the result:  
3  
Note that only vaild expressions will give you a result, else you will just get an parse error!