Hi SenecaLakeMonster and welcome!
Why are you using attributes? If you don't need to constrain yourself to a published external schema, I'd do away with those. Just a pain. Elements are fine for everything.
Now, to your question ... you cannot set the value of a specific row in a list using out-of-box rule-actions (because there's no "filter" button on the target field dialog).
However, you can manually edit the manifest.xsf to do this but it won't work in the browser.
If you need browser support, qRules SetValue command does the trick. That's why we created it. Go to Qdabra.com and download a free trial and you'll have it working in no time.
Slime,
P