I am using OpenERP 7.0 installed on our own servers. I managed to create campaigns by going to Sales -> Configuration -> Leads & Opportunities -> Campaigns and I found out how to filter my leads by campaigns, as detailed in the documentation. However, I can't find how to associate the lead or opportunity to a campaign in the first place: there is no "campaign" field on the lead or opportunity form and it's not in the "categories" field either.

What do I need to do in order to be able to assign my leads and opportunities to particular campaigns?