I was having this problem forever, and here's what you do to stop it. There's 2 ways of doing it. #2 worked with me, but #1 didn't but its different with different people.
1) When you are at the activate part and have to enter your email address, enter the one that is linked to your phone (to find that one out, on your phone go to Main>>Settings>>General>>Phone Information then scroll to the bottom to find the email address linked to your phone.
2) If #1 didn't work for you (like it didn't work for me), do what I did to make it work. Uninstall the version of Media Manager that you have on your computer. Go to
here (<<click) and click More Details and then download. That is the latest version of the Media Manager. Afterwards, on your phone go to Main>>Settings>>General>>Update Data Profile. Once all of that is done try again. It should work. If not, on your phone also Update Software and Update PRL.
Tell me if this works.