Newbie – Internal linking problem

Hi,

I have the latest version of wordpress and have made a post. The post can be seen here:

http://www.geohacking.com/

Notice after the first paragraph there is a nice clickable link to the rest of the post: “Read the rest of this entry »”

However, on the right hand nav section if you click “Mapping” in the categories section you will see a portion of the post with an “[…]”. But there is nothing to click to see the rest of the post. Shouldn’t there be an automatically created clickable link? If not, how do I create one for this section?

Thank you in advance.