The nightly builds work only on Mac OS X 10.7+ Intel 64bit. Mac OS X 10.5, 10.6 and PPC, Intel 32bit 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