Call up your reservation on the KLM website using the 6-character alphanumeric passenger record locator (PNR). This should be visible on your confirmation and e-ticket; if you bought your ticket from a travel agent, you may have to contact them to obtain the KLM passenger record locator.

When viewing your reservation on the KLM website, in the section entitled "Your baggage", there should be a clickable button saying "Arrange extra baggage".

Do you see this, and what happens when you click it?