This is usually because you’re trying to display posts from a personal Facebook profile. The plugin can only be used to display posts from a public Facebook page. The easiest way to check whether you’re trying to display posts from a personal profile instead of a page is to view your page and see whether you have ‘Friends’ or ‘Likes’. Profiles have ‘Friends’ whereas pages have ‘Likes’. If you have ‘Friends’ then it’s not a page, it’s a personal profile.
If you’re using the profile to represent a business, organization, product, public figure or something else other than yourself, then we’d advise converting your profile to a page per Facebook’s recommendation: http://www.facebook.com/help/175644189234902/, as there are many advantages to using pages over profiles. It is in fact against Facebook policies to use a profile to represent something other than yourself (see ‘Why should I convert my profile to a page’ in the link above).
Once you’ve converted to a page then the plugin will be able to retrieve and display all of your posts.