First off, Google Hangouts requires a plug in for browsers other than Chrome:
If using chrome, you can download the google chrome extension for convenience:
If you cannot find an application you want to use:
Google Hangouts has two options to download apps. The first is the icon on the left of the hangout screen. The second is if another user in your hangout uses an application, you will have the option to download it. If the application you are looking for is not possible to download via these two methods, then it is not possible for you to download that application at this time.
If no one can see or hear you:
Check your settings to make sure that your microphone and webcam are on and working.