Payload is just a folder. The .ipa file format basically consists of a zip file made of a compressed folder called Payload containing the .app file. Airsign is a 3rd party codesigning tool. It redoes the signing that Corona does when it builds your project for iOS, but it somehow does something slightly different that supports the Apple Testflight Beta testing.
Here’s the bash script that I use to make .ipa files from the .app file Corona produces when it builds an app.
dir=$(dirname {query}) cd $dir mkdir ./Payload mv {query} ./Payload file=$(basename {query} .app) zip -r $file.ipa ./Payload
{query} is a parameter passed in by Alfred and is the path to the .app file. So usually /User/user/Desktop/MyGame.app. If you replace all the references to {query} with the path to your .app file and run it in Terminal, you should wind up with a .ipa file.
Edit: in fact I think the script can be made Alfred neutral by doing the following:
path="/Users/username/Desktop/MyGame.app" dir=$(dirname $path) cd $dir mkdir ./Payload mv $path ./Payload file=$(basename $path .app) zip -r $file.ipa ./Payload
If you install Airsign, set it up, and then open your .ipa file with it, you can find options to sign the .ipa with the AppStore provisioning profile you were using to build with Corona. Do that, then there’s a button in Airsign marked ‘Save as .ipa’, click that and then upload the new .ipa file made by Airsign using the Application Loader as usual.