Apologies for the delayed response!
I am glad to hear you enjoyed the user group event! I have now uploaded the file with my personal steps that I had prepared as a run-through the two challenges. The link is here: https://community.tableau.com/docs/DOC-18185 and I think I had written down two different types of calculations that I had come across - both can be used to create the up/down arrows.
Let me know if this doesn't help.
Thanks very much Elena