1. Can I adjust audio volume with JustStream while streaming to an Apple TV?
No, you can’t. The app streams media by means of the AirPlay protocol, and due to its limitations, volume can only be adjusted on the target device itself. You can normally do this with your Apple TV’s remote control.
Note that the above applies to all devices that communicate via the AirPlay protocol, and you also cannot adjust volume when streaming a movie from an iOS device such as an iPhone or iPad.
2. Is it possible to stream audio from JustStream to AirPort Express compatible speakers?
No, currently this functionality is not supported.
3. By using JustStream do I have to install anything on my Apple TV?
Being a desktop app, JustStream should only be installed and activated on your Mac. The AirPlay protocol is built into the Apple TV, so there is no need to install anything additionally on it.