Thanks to both joelwarren and bryanobx. I was able to get this to work using your suggestions.
And here’s one of my own:
If your attributes and values contain multiple words, the ‘+’ feature only partially works. The URL parameters you use ARE case sensitive, and the lowercase translation does not work as you are led to believe. According to the description, this should have worked:
yoursite.com/product/yourproductname/?attribute1=word1+word2&attribute2=word3+word4+word5
I had to make the following changes to the URL to make the plugin work:
yoursite.com/product/yourproductname/?attribute1=Word1+Word2&attribute2=Word3+Word4+Word5
(notice the capitalized 1st letter of each word)