The button was actually removed, presumably by accident, during a software update a couple months ago. You're not the first one to report it but as of yet it has not been added back and may not be in the immediate future. The only workaround for the time being is to add it if you see the product in another member's I Want It or I Got It list in his/her profile. If anyone else asks you, though, be sure to tell them you found it without a problem.
I feel like an idiot. I just got a new Zune and I can't find the button to add it to My Products. Help!