Let me answer you how you can join a signature campaign and a little bit about the campaign that you joined as per the link you included in your question.

First, you will be able to join a signature campaign once you become a Jr. Member. Then, you may look for campaigns and ask to join. For example, in the link that you included above - I cannot open it now because it is already full - but I am sure that there will be instructions on how to join in the campaign. It involves replying to the forum with the details they are asking and wearing the appropriate signature applicable to your rank. Wait for the moderator to accept you. Once accepted, you may start posting and replying to the forums. From the Home tab, just find a topic you want to reply to. Bear in mind to read the link of your campaign because each campaign have their own rules. Some examples may include specified number of posts required per week, number of characters per posts, which posts shall be counted and not. Failure to do so may result in your efforts to be useless, be removed from the campaign, or have no pay.

Let me suggest that you really involve yourself in the forums and read through them because that is one sure way for you to learn the way around. Also ask help from whoever shared bitcoin signature campaigns with you!