• Go here: http://www.tumblr.com/customize/
• Go to Pages
• +Add a Page
• Do whatever you want to it
• Save Page
• Copy page link (which is something like http://YOURTUMBLRURL.tumblr.com/PAGENAME)
• Insert it into your links, if theme has that option.

To manually add a link, use this code:

<a href=”http://YOURTUMBLRURL.tumblr.com/PAGENAME”>PAGENAME</a>

and paste it somewhere, like in your description.