How to open Sibelius files in MuseScore
Want to transfer your sheet music collection from other music notation software? Use MusicXML.
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/