Dude, don't use that - it's not worth it at all, and it screws up your CTR with Adsense.
I unwittingly used "plug a link" a few times - then I sort of realized that there was something fishy about the whole thing; so I asked staff about it, and learned that it's jacked up, and quite using it.
Here's the sites that I back link on that work for me:
7. Best Reviewer
8. Google +
9. Google bookmarks
10. Live Journal
11 Soda Head
12. Article Blast