Ok it seems easy enough, install, enable, add some text for both types of users, make sure exclude is off, save and it should work right?

Err No.

Am I missing something?

Assuming you are using standard Joomla content, that is all that should be needed. The only other parameters that may need checking are "Only display footer in article view" (set to yes, and the footer is only displayed when an article is showing), "Add footer to article" (this defines how the footer is added - if yes, the footer will be appended to the text of the article, otherwise it'll be added outside and will depend on other plugins), "Exclude footer from uncategorised articles" (set to no and the footer will be shown on all articles).

Are you using any other content management components?