With Facebook’s shift to its new “Timeline” style layout, it's not as easy anymore to add sign up forms, but here a guideline:
If you haven’t worked with or installed iframes before, click here to add the Static HTML app to your page.
Once it's installed, you need to choose the Page that you want to add the application to.
After you’ve done this, the app (labeled as "Welcome") will then be displayed with a star icon, positioned next to any other apps you might already have.
Next, click on the big star to access the app.
You will see a blank box appear, which is your canvas for entering whatever you want onto your iframes tab. Essentially, anything you enter into this area is your “application”.
So for example; this could be a form, a couple of links, an image or even just some plain text.
All you need to do here is drop in the embedded code for your subscription form.
Click “save” on the top right hand corner, and the app will then appear on its own tab on your Facebook page.
(Note: You can also preview your app before you save it, to make sure you’ve entered the proper code and that it will display correctly).
Now that you’ve got your subscription form, it’s useful to do some editing on the iframes tab so that visitors will be able to identify it easier.
Backtrack to your Timeline and look at the Featured Apps area. The subscription form app should now appear in this list.
Click the Edit icon for this app and go to Edit Settings.
Edit the name of the app by calling it "Sign-up to our Newsletter”, or anything that would appeal to someone viewing this page.
You can also upload an icon for your new app tab. When you’ve done that, click save and then click “Change” for “Custom Tab Image.”
And that’s all there is to it. Your custom subscription form will now display on your Facebook page in the app tab area with the new title.
To finish things off, go back to this area again and click the Edit icon as you did before.
You can then change the order of your apps to place your subscription form in a better location - in case you already have a few other apps taking up the prime real-estate.