If you are running Arch Linux, you can install Mopidy using the mopidy package found in AUR.
To install Mopidy with all dependencies, you can use for example yaourt:
yaourt -S mopidy
To upgrade Mopidy to future releases, just upgrade your system using:
yaourt -Syu
Optional: If you want to use any Mopidy extensions, like Spotify support or Last.fm scrobbling, AUR also has packages for several Mopidy extensions.
For a full list of available Mopidy extensions, including those not installable from AUR, see Extensions.
Finally, you need to set a couple of config values, and then you’re ready to run Mopidy.