Lotus notes dropdownlist is created with predefined list items.I have updated the lotus notes document using a VB6 program and populated the field.
However when I tried to view the value it shows empty.(though the values are fetched successfully in the vb6 program)
If I'm reading your question correctly, you have a dropdown field on a form, and you specified the allowable values directly in the field properties. (Since you haven't said that you are using any other technique, such as doing a lookup from a config document, this is my default assumption.)
Now your VB6 code is setting a value in the NotesItem that corresponds to the field on your form. You have not said whether or not the value that this code is setting matches one of the choices defined in the field properties on the form. I suspect it doesn't, and I also suspect that you do not have the "Allow values not in list" option checked in the field properties dialog on that form in Domino Designer. If that's true, I believe that will cause the Notes client to display an empty field even though the VB6 code set a value.
The other possibility is that your VB6 code has misspelled the field name, but without seeing your code, a screenshot of the form, and screenshots of the field properties dialog tabs, I can't know for sure.