Hello Patrick,
Thanks for the reply! I will clarify my question so everyone can understand what the issue is although I've resolved my problem already.
Someone open the form, fill it out. No problems here. Now the person saves the form to sharepoint and then start an approbation workflow. An approver recieves an email to tell him he needs to approve the form by signing it. So he opens the form. In the approval section, he should see his name. Under his name is the link to sign (Click here to sign this section). He clicks it to sign the form, select his certificate, check the check box and click sign. This is working fine since the workflow continues after he has signed. The problem is, he has signed, but his signature doesn't show. Actualy, it was working when I first published the form, but I had to make changes to the Data source of the form and it is then that it stopped showing the approvers signatures.
What I had to do to resolve this problem is create brand new sections, copy/paste what is inside the old sections into the new ones and set the signatures on these new sections. Then the signatures started showing again.
Hope that helps someone else someday!