Currently it is a bit complicated to download the compiled ipa file. It will be simpler in the first version.
Basically you need to:
1. Switch to a store key instead of the developer key.
2. Compile your app in release mode.
3. Download the ipa file from the builder.
4. Now you need to follow the standard publishing instructions. One of the steps requires a Mac. Either a local one or MacInCloud service. The Mac builder option will not help here.