[Mac] Cannot change all score files to open stable version of MuseScore

1. Right-click MuseScore score file.
2. 'Get Info'.
3. Click 'Open With'.
4. Change application to MuseScore 1.0 (it currently opens using the nightly build).

Expected result: Dialogue box appears asking me if I wish to change to 1.0.
Actual result: Dialogue box appears asking if I wish to change to the nightly build.

Using Mac 10.6.7.