As you can see from
my post earlier in this thread, I had the same issue. Eventually, I got permission to include a URL in my sig. I didn't do anything special to make it happen, and I never found out exactly what the criteria were for getting permission. I think it's a combination of number of posts, and time since signing up. It's also possible that reputation points help, and/or that moderators are inspecting our posts and using a bit of professional judgement.
I don't actually mind that we can't use links until we get permission, because I sure would hate to see spam ruin our forum. I just wish we could get a clear statement of the rules, so we would know how long we're going to wait.