BuddyPress.org

Can you show me where i should add this?

You can’t just add the code ‘as is’. Because you’ll need the ‘mytextbox’ class on your input elements as mentioned here:

Where mytextbox is a class belonging to your text input fields.

You should check out these links:
http://www.w3schools.com/html/
http://www.w3schools.com/css/

The tutorials will show you how to do the sort of stuff you’re looking to do quite easily.