Error :java.lang.ClassNotFound Error: jxl/format/CellFormat --NWDI

Hi all..

           I am doing Download to Excel Functionality. For that i added jxl.jar file into my external jar file. But in run time i am getting this exception..

1. java.lang.NoClassDefFoundError: jxl/format/CellFormat

for this what should i will do? anybody explain to me..

it's very urgent..

with regards,

Pradeep