To be able to open encrypted documents with itsme, you must first and foremost be registered with itsme. If this is not yet the case, first consult www.itsme.be and / or download the itsme app on your smartphone.
When your registration with itsme is complete, you can link your itsme to your Doccle account. See the FAQ How can I link my itsme to my Doccle account?
Once itsme is linked to your account, you can also use it to log in to your Doccle account, open encrypted documents and encrypt other documents yourself.