I've downloaded the iOS 5 beta and want to test my app, but since I don't want to install this on my iPhone, I'd like it to be only installed on the Simulator. Does anyone know how to do this? In the Xcode Organizer, the software image shows up, but I can't run the app in iOS 5.
You cannot install or update iOS software/firmware for iOS devices on a Mac because of the different computer architecture.
Download and install Xcode 4.2 with the beta iOS SDK, then you can test the app on the iOS Simulator.