How do I add an active URL to my signature?

Select Profile

Select Edit Profile

Select Personal

In the signature box, type your URL like this:
[  a href="http://www.yoururlhere.com"  ]YOURBLOGNAME[  /a  ]

(but don't but the spaces before or after the brackets [   ]  (I put the spaces in for demo purposes)

Click save personal settings.

So you, personally would type:
[ a href="http://marcylana.tumblr.com/" ]Marcy's Garden[ /a ]  (without the spaces)