Here is a link from the Learning Center that gives great detail on how to insert links into your Hubs via the "Link Capsule": http://hubpages.com/community/Learning-Center-link
Another way to add a link is within the "Text Capsule" by highlighting a word while you are in edit mode within the capsule. In the top of the box there is a "chain link" icon. If you click on it it will allow you to either type in a web address or copy and paste a link. When you save your work, the word will appear blue and when you scroll over it, it will be underlined so that the reader knows that if they click on it, it will take them to another page.
I hope this helps answer your question!