The edit below made this workflow functional for me on Mavericks. It changes the referenced version of Ruby from the default (2.0) to 1.8, which works with this script.
Select the workflow. Double-click on each workflow item to edit it (the pair and next Script Filters, and the two /bin/bash scripts), and make this change in each:
Add songs to iTunes Up Next
in Share your Workflows
Posted
Just wanted to say that this worked perfectly. Big thanks gena!