After digging around various threads I figured it out:
Install the "loadpaths.rb" script (http://forums.sketchucation.com/viewtopic.php?f=180&t=29412)
into SU Plugins. I modified it with:
$LOAD_PATH << "#{pre}/gems/1.8/gems/ruby-ole-1.2.10.1/lib"
$LOAD_PATH << "#{pre}/gems/1.8/gems/spreadsheet-0.6.4.1/lib"
Backup and clobber the msvcrt-ruby18.dll with the one from the ruby1.8.6 release.
require 'spreadsheet.rb' -> true
-Robert