To enable itunes plus, you just need to go to account control pannel, then go ahead and click the "Manage iTunes Plus" button. On the next page, make sure the box is checked next to "Always show me iTunes Plus music and music videos when available”(this checkbox also can be used to disable or turn off iTunes Plus)and click "Save Changes".
Now that iTunes Plus is enabled on your account, you'll now be able to buy DRM-Free iTunes Plus songs from the iTunes Store. any music and music videos which are available DRM-free will show a plus (+) sign next to the price, which is $1.29 for each track.