Hey Enzo,
I am really sorry for the delay of this reply as it is actually a public day off for us here. This is what you will need to do:
1. Register on Mollom for an account to generate the api keys
2. Edit the custom field app "mollom" by accessing Applications > Custom fields. When you are editing the app, you will see this screen,
http://screencast.com/t/lj1iYsGc
3. Enter the api keys generated from #1 in step #2 and add the mollom field in the profile type
By the way, please remember to assign your domain to your license to obtain for support in the future. You can do so by accessing your license area at
http://stackideas.com/dashboard