The aiStarter program is installed on your computer when you install the App Inventor Setup software. It provides communication between App Inventor running in the browser, and other parts of App Inventor. Whenever you want to use the emulator or the USB cable, you need to make sure that aiStarter is running. If it is not running, you'll get an error message saying that aiStarter does not appear to be running. On Windows, there should be a button on your desktop that starts aiStarter. On the Mac, aiStarter should start automatically, so Mac users should not have to worry about this. In case aiStarter somehow does need to be started manually on the Mac, find it using Spotlight, use the Finder to go to /Applications/AppInventor/commands-for-App, and double click on aiStarter.
aiStarter is running correctly if the emulator (or USB) connect. If you want to diagnose a problem with aiStarter, try running aiStarter from the console.
Then, when you try to connect the emulator, the console output for aiStarter should show a sequence of "probes" of the form
127.0.0.1 - - [01/Dec/2013 11:28:30} "GET /replstart/emulator-5554 HTTP/1.1" 2000
There might also be problems, like a message about blocked ports, or issues with the adb program. For a blocked port, check whether there is another program on the computer that is using the port.
The adb (Android Debug Bridge) program runs on your computer, and controls attached Android devices.
The the program can sometime get confused, especially if there are other issues with connections. If you suspect a problem, open a console in the Commands to App Inventor directory, run the program adbrestart and see if that helps.