Christosz,
Click User CP. It's on the left, next to chat, blogs, FAQ...
Under Control Panel, go to "Your Profile".
Click Edit Your Details.
In the 4th box "Additional Information" there is a place for you to change lbs. to kgs. It says "Use ___ for weights. Click the blue arrow to make your selection.
Go all the way to the bottom and click save my changes.
I hope this helps.