Board Thread:Support Requests - Getting Technical/@comment-5506946-20130401004831/@comment-10688548-20130505165128

You must have enabled achievements for the AchievementsCustomize page to be available. Achievements are enabled separately, via the Special:WikiFeatures page.

I thought I would throw in a note for the OP: big wiki-boys and wiki-girls should be able to go to a special page by manipulating the URL, without having to have a link to click on. However, there is indeed at least one link to AchievementsCustomize: once you enable achievements, it can be found on the "More Badges You Can Earn" section of your userpage.