Facebook have just made a change where you need to create your custom page tabs and attach them to your Facebook pages in a slightly different way.
It is actually very straight forward and we are in the process of integrating this directly into my Easy FanPage design plugin, however until this is done you will need to do the following:
For every new page tab you will need to visit this URL: http://www.facebook.com/dialog/pagetab?app_id=YOUR_APP_ID&next=YOUR_URL where YOUR_APP_ID is the app id of your app, and Your URL is the app where your custom page tab html code is stored. In the case of Easy FanPage Design it will be something like this: http://justinwheeler.net/fanpage/Hello-Tab.
To make this simpler you can go to http://www.easyfanpagedesign.com/add-pagetab-to-fanpage.html or you can read on below:
Once you have done that a dialog box will pop up that looks like this:
You should then choose the page you wish to add the tab to and it is done.
If you are confused by any part of this process you may find the below video useful:
TROUBLESHOOTING THE API 191 ERROR
Sometimes you may encounter an API 191 error when adding a tab to a page. This is invariably because you have got the syntax wrong.
It is very important that your URL is the exact same URL as you have on your Page Tab in Facebook Developers and also that this URL is the same URL as you have as your Direct Link to the Custom Tab template.
If they do not match ie one is http://www and the other is http:// then you will get this error.
Do let me know how you get on.