I know it sounds silly but I have been trying to find the lever or something to open the bonnet of my mercedes A-Class.

Just do as Silver said.

Check your car owner's manual. If you don't have the manual, check this website for a free online owner's manual:

http://www.edmunds.com/ownership/parts/articles/120189/article.html

First, pull at the release lever under the dash. Its usually red, so you wont have trouble finding that...

Once you pull that, the bonnet jumps up a bit, and the bonnet release tab pops out from the grill. Its a small black colored tongue. Pull it and the bonnet releases.

Note: This is the standard way for the Mercedes. I'm not sure if its any different for the A Class.