Hi Shobit
As per your previous reply i could get the message ID and message no. but these are not some thing related to the Pop-up.
I have used the "Message replacement" method and still i could not change the text of the pop-up with the required message because both are not the same. 
Can you help on this? Basic criteria is to change the text "Required entry field not filled. Navigate anyway?" to a customizing text on the pop-up.
Thanks
Chinky.