Once installed, Shotwell is available in your Applications menu under Graphics or Photography.
Shotwell may also be executed automatically when a camera is plugged in to your computer. To check that your system is set up to run Shotwell when a camera is detected, go to Edit ▸ Preferences in any Nautilus (file browser) window and choose the Media tab. You'll see a dropdown box entitled Photos: which lets you choose Shotwell as your photo handling application.
Shotwell may also be executed directly from its build directory, although this is only recommended for testing out Shotwell or for developers.