Here's a walkthrough on how to work with some of the restrictions during the current developer sandbox phase that Kyle pointed out here.
So let's assume you've gotten on the whitelist, logged in to the developer site, started creating an app and you're at the point of saving your chunk of code (for a walkthrough of that process, see here )... now how do you actually see your app live?
First you need to install it by getting to the App Profile:
An App Profile (see "Anatomy of a MySpace App" for a more complete explanation) is your app's very own MySpace profile. It's automagically created along with each app. Most importantly, it's the *only* place where anyone (including yourself as the developer) can install the app.
To get to it from the Application Builder, click on the "Edit App Information" link on the left nav:
From the Edit App Information page you can click the "Visit Profile Link"
From the app profile you can click the "Install Application" button and the page will redirect you to the app after installing.
Great! Now you have your app installed and
YOU can see it… but how come no one else can see it!?
All apps are hidden from viewers unless the viewer is friends with the app's profile during the developer sandbox phase. Likewise, App Profiles are private by default. Keep in mind that the app’s friends are not the developer’s friends. Remember your app has its OWN distinct profile! As the developer of the app, you're automatically the app’s first friend and that's why you're allowed to see the app.
Thus, in order for another user to install or view an app, they must be:
- logged in to MySpace (even if the app is on a publicly viewable profile)
- a friend of that app
So the natural question that follows is: "How does my app add friends?"
Well you can actually login to MySpace as your app. Remember the email and password you entered when you first created your app on the developer site (on this very first page)?
Hopefully you have them handy... just use those credentials to sign back in to MySpace as your App Profile:

Now you're logged in under the app’s profile and can add friends:

To add a someone as a friend of the app, go to their profile, for example:

Click "Add To Friends" and continue to follow the standard MySpace procedure for adding friends.
Once your app's new friend accepts the request, she can install the app (if the three installation limit hasn't been reached yet!) or view the app on profile's where the app is installed.
Understandably, some of the above can be somewhat cumbersome and confusing, however in the future this “private” state is fundamentally designed to allow your app to be in “stealth” development mode, while still allowing for your app to be shared amongst a controlled set of users.
-- Chris