As of March 16 Facebook changed the way custom pages can be displayed.
Here is what you need to do in order to post a custom page / custom tab. It looks a bit complicated as it is meant to provide a rich application environment and has a lot of overhead for just ading some custom tab. However if you follow these steps you get it done in a few minutes. Feel free to use the Social Media Academy test page to test your tab.
1) Go to the developer section and create the link between a page / content you are hosting somewhere on your own server and the facebook page you like to show it on.
Click on "+ setup new app" on the upper right hand side
2) Give your application a name
3) Click on Facebook integration (tabs on the left hand side)
Add a name under "canvas page" for instance : promopage
4) Add the URL where the content resides
For instance if you like to display something from http://socialmedia-academy.com/demo/index.aspx
You will need to add http://socialmedia-academy.com/demo/ (note that you will have to ignore the actual file name)
Call the content file index.aspx (not htm and not html)
5) Scroll down and give the tab a name
6) Add the same URL i.e. http://socialmedia-academy.com/demo/ as the "Tab URL" — And submit
7) Click on "Application Profile Page" and on "add to my page" on the left navigation.