About Listview Control

Hi!,
Please tell me how can I add/enter text at run time in listview control (as sub items).
i.e. I have 4 columns in Listview Control
No. Item Qty Rate

when I select P.O. 1 from combo box then it displays No. Item and Qty in listview, I want to add Rate at runtime.
I could not enter the Rate in Rate column how can I do this?

Thanks.