As a guest, your "Karma" is essentially zero. If you register and post as a wikidot user instead of a guest, your karma starts out at zero and then gradually increases. The little colored bar next to people's names shows their karma level.
Wikidot has been having a lot of issues with people spamming sites, so they have added a new control. If a users karma is 0, or they post as a guest, then they are not permitted to post links. Once your karma reaches 1 (a single bar) then you can post links.
I could turn that off, but it is not a granular control. I can't (for example) turn it on for you and off for another guest. I'm still considering the ramifications of leaving it on vs turning it off.