hey yoluca,
when you're at your easysocial backend go to profiles, then select the profile type that you want to use the easydiscuss signature with.
then click on the "custom fields", on the right side you will see all sort of field types that you can use on your profile, one of these is "discuss signature" - just drag and drop it to the position in your profile where it should appear. http://screencast.com/t/lpbtoEiz
your profile is now ready to display and edit easydiscuss signature for fine-tuning you can click on the field itself after you dragged it into your profile and modify some settings, for example if it should be appear in the registration and so on...

hope that helps