If the install software won't run, it could be becasue your 
disk permissions are corrupt and need to be repaired.

Run the "Disk Utility" software located here:
Applications/Utilities/

Select the "First Aid" tab (you may select the volume to be repaired as well)
and click on "Repair Disk Permissions".

It could also be because you have have Stuffit Deluxe or Standard 8.0.0.

The problem is not with the with the Roni Music software but a
problem with your version of Stuffit Expander.

Either download the Stuffit X version of this file here:
http://www.ronimusic.com/download.htm

or go here:
http://www.ronimusic.com/stuffitbug.htm
and read how to update your version of Stuffit.