Note that it's just my preference to put 'bot-specific directives before catchalls, and I don't use the Allow as it's non-standard. In this case, of course, the Mediapartners bot would obey it; I just don't want to keep track of which 'bots obey it or not.
I have the same issue and I just followed the guidelines above. How long does it take to show results? I just launched a website and initially I just blocked Googlebot but then I did not see any AdSense ads. Now I have allowed MediaPartners-Google and when can I expect to see ads?