If you have a link to your site in your profile, google will pick up the link in the drop down menu. And if your profile is visible by the public, google will pick the link in your profile.
Go through the forum and make some quality post.
Then go to the next forum.
Because there is no link in your signature and no link in your post, very few moderators check peoples profiles - especially on busy forums.
If you find a forum with 15,000 - 20,000 + members, and a forum that gets a few thousand post a day, its very easy to fly under the radar, build up backlinks and never be noticed.
When doing this type of link building, I mainly target VBulletin forums.
When you sign up on the forum, use targeted keywords in your username. Such as my name here is "Texas Hiker" and a link to my blog is in my profile. That is because I want the keyword "hiker" associated with my blog and I am from Texas.
2 - 3 weeks ago I targeted a popular forum with this same technique. And the links are already showing up using the link: sitename command in google.