You will get all the instruction here if your read it thoroughly. http://forums.digitalpoint.com/faq.php
When you have reached the requirements you then need to click forum actions at the top of the page, select edit profile and then edit signature
When I first joined it was 10 posts, and waiting 7-10 days. I see they've upped it, but yes there is a requisite amount of activity that you have to put in before you can get a signature.
Seems like a long wait (for someone that's just joined like myself!) but then on other forums (which shall remain nameless!) I am tired of every thread having hundreds of 'Great post!' replies just to get their links in there so I think it must reduce drive-by spamming quite a lot.
please read the forum rules and go to control panel there is a option edit signature where u can edit your signature.