Positive:

Enter proper pincode and click on OK (or press enter key, 
here default value would be Ok button when enter key is 
pressed)

Your second window should display the corresponding post 
office address.


Negative testing
1.Do not enter pin code,and click on OK.then it should give 
an error msg asking the user to enter pincode.
2.Check for the min & max length property(usually pin code 
length is fixed) ,
3.This field should not accept values other than numerics 
(ex: check if it is accepting "space as a character", alpha 
values, special chars).

Thank q........ same thing i told to interviewer then he 
told you forgot imp test case...........

any way thank q very much..........

before that u have to write d testcase for textbox that
cursor is in text box r not..
OR 
to check for lenth of d string u go for boudary value analysis
u may know m-1    n-1
             m      n
             m+1   n+1

I think first u have to check the existence of the text box
so first test case should be if text box exists.

1.Open corresponding window.
2.After opening corresponding window the cursor should 
highlight on pincode text box.
3.Enter valid pin code i.e under certain limit.For valid 
pin code it should highlight OK button.
4.For invalid pin code it should display error message 
box "Enter valid pin code".(i.e due to wrong code/due to 
entering code above limit.)
5.After valid pin code the cursor should highlight on OK 
button .
6.Click on OK button,the post offices address window should 
open.
7.The post office address window is not opened due to 
misclick of OK button.

Also for a blank pin code