You must have your app already installed on an iOS device before you can trigger any functionality using iBeacons. There is no way to have the presence of a beacon automatically prompt a user to download your app from the AppStore, no matter what the user interface.
The only exception to this is that if the iOS device already has another app installed that prompts a user to download your app. You could even do this with a pre-installed app that responds to beacons like Passbook. But even in this case, you somehow have to get the third party app developer to put in custom code to link to your AppStore page on iBeacon detection, or somehow get a user to put in a new passbook entry for prompting to install your app.
The bottom line is that all of these exceptions are generally harder than getting people to install your app via other mechanisms.
Sorry.
EDIT: It seems that lots of reports of this are really referring to the new Suggested Apps feature of iOS8. This is an AppStore feature that uses geofences (not beacons) to find apps relevant to your location. The description of this feature in settings says:
Leaving this option turned on will allow iOS to offer suggestions for App Store apps that you do not already have installed. These can be based on what you already have installed or what people near you find interesting.
Again, this is based on geolocation not beacons, and it is unclear whether the suggestions are based on an algorithm (other folks regularly using an app in the vicinity), a database (known locations of major retailers), or paid marketing (e.g. Starbucks pays Apple to suggest their app when near one of their shops.) It could be a combination of all three.
How do I know that this technology is not based on beacons? Because I received an app suggestion for Starbucks (without having the app installed) and then I immediately used a non-iOS-based beacon scanner that verified no beacons were in the vicinity.
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