I see that I can set up product extensions in my account now. I already have my Merchant Center account tied to the client's AdWords accounts. And when I go look for the ads, there's a link that says "Show products" for the products I've enabled. This is all great.

However, some of the products aren't matched properly (and I looked at some of my competitors, and theirs are even worse - lucky for me!)

So I got an email from the AdWords Blog announcing these were available to everyone, and it says something about controlling which products display with which ads by "making a simple addition to your Merchant Account" - but they don't say what the addition is! Where is it? Is it in the feed somewhere? If it's in the account, where is it? All I can find is that the two accounts are tied, not that I can specify products to an ad or ads.