The nightly builds work only on Mac 10.6+ Intel. Mac 10.4, Mac 10.5 and PPC are no longer supported.
The nightly builds allow people to test for bugs, features and fixes in advance of the prerelease and stable versions. For a more detailed description, see Comparison of stable, prerelease, and nightly builds. A nightly build can run side-by-side with other versions of MuseScore on the same computer. If you change preferences in a nightly build, it does not affect your preferences in the prerelease, or stable versions of MuseScore. The nightly builds use a cymbal logo to visually differentiate them from normal versions of MuseScore (it is also a play on the fact that they are likely to crash). If you perhaps find this overwhelming, then it is recommended to use the latest stable release from musescore.org.
* Current nightly builds are generated from new development code and are not stable.
You can subscribe to the nightly RSS feed