To begin working with applications in Facebook, you need to set up your account to work with the right tools. To do this, click Developers at the bottom of the screen, then Get Started. Next, click Add Facebook Developer Application to include the app in the list under your profile.
In your profile, under posted items, you'll see applications you've created using a third-party site, such as Popfly. Click one - in this case, ImageRotator - and you'll see various details of the application (including a URL that you'll need in step 5). Click Post to add the item to your profile so that you can start to work with it.
Next, select Developer under Applications on the lefthand side of the screen. The My Applications list on the right shows the apps you're currently working on - it should be empty. To get started on your app, click the Set Up New Application button at the top of the screen. This will take you to the New Application page.
Here you can enter various details for your application to assist those who wish to share it. You have to enter a name for it, as well as an email address so you can provide support. Once you've finished, indicate that you've read the terms and conditions for using the Facebook Platform and hit Submit.
To work on your app at any time, go to the Developer page and click See My Apps, then select the edit link above your application. This will reload a similar page to the New Application one. Enter the URL of your mashup (provided back at step 2) under the entry Installation Options, Post-Add URL.
Once you're finished, click on your app's name (from the Developer page). Click the button labelled Add Application to add the app to your Facebook account. Also on this page are links to start discussions of the application (for example, if you want to pass on development news) and reviews from other users.
When you click the Add Application button, this will display the add screen for that particular applet. To include this in your profile, you'll have to allow the application to know who you are, but other functions are optional. If you add information about your application, this will be available from the about page link.
The final step is to get people using your app. On the Developer page, select your application and click the Share button. Select Send a Message to start communicating with friends - once five of them have added it to their profile, you can click the Submit Application to share it more widely.