itunes match not recognizing computer

I purchased a subscription to itunes match in order to convert AAC protected songs to mp3. I still get the message that the song can't be converted because it is AAC protected. When I go to the store in itunes and click on itunes match, I am prompted to "add this computer". I have clicked "add this computer" every time I go to itunes match. Is there something I am not doing?? Why am I still unable to convert these files that I purchased in itunes years ago?

Thanks!