Log in to your Instapaper Account screen and you’ll find the ability to link your Instapaper reading queue to your Readability account.
As a side note, last week I mentioned how the sites I most want to support are usually not the sites I’m throwing into Readability. But after using Readability for the past few days, I learned that not just sites you read using Readability are given a donation, also the sites which you simply bookmark for reading later. So now, if I read an article in the browser, I just bookmark it and it gets earmarked.