How do I get a API for cause- or communitypages

Simon,
An Application has to be created by a real person, it can't be created by a Page User. An application does not automatically post to, or interact with, your personal Facebook profile at all. You are simply the administrator of the application and can configure the application. That's all. You can still use your application on your Facebook Page (as a Page Tab) and do other things like that.

Hope that explains. This is Facebook's policy and restrictions. They want legitimate people to own everything on Facebook, just like your Cause Page is owned by your personal profile (I'm assuming) as well.

Alex