I am unable to modify the content of current web page in chrome extension.
I need to parse through the contents of current web page and append data to it.
Please Help....

Thanks in advance

This sounds strange.
I have no problems at all accessing and traversing the DOM in my content script.

Try something simple at first, like finding a button or something using it's id:
var btn = document.getElementById('qr_submit');

and change the text:
btn.value = 'New text';

(This example would have worked on this page. The 'qr_submit' button, is the button "Post Quick Reply" below... I picked that example, since I have no idea what the page you are trying to modify looks like..).

Have you verified that the content script gets included when you load the target page?