that’s easy. When you are in your profile, there is a separate link which says ‘Add Certification/Training’. When you click there, you first have a choice of organization through which you are certified, and then a further subcategory of the type of certification. Then you just fill in the fields as required, and save.
It will get into the status ‘pending’ until the organization has verified your certification. Be aware that not all organizations are verification partners in which case I would get on their nerves until they become one :-).