How to open Sibelius files in MuseScore
MusicXML is the standard exchange format for sheet music. Sibelius users can download a plugin to export their sheet music to MusicXML files and then import these in MuseScore via File -> Open.
Sibelius 7 has onboard MusicXML export, but Sibelius 1 → 6 requires a plugin
You can download the Dolet plug-in, for Windows and Mac OS X from: http://www.musicxml.com/dolet-plugin/