A lot of teachers find it helpful to have a class resources section with links to external sites or uploaded documents.

To create a link to an external site or document, click the last icon on the row of text editing buttons - it looks like a little globe.  Where it says "Text Link:" type in what you want your link to say, for example "Assignment Worksheet".

To upload a file, click Upload Files on the bottom and find the file on your computer. Wait for it to finish uploading and it'll show up as the selected option in the dropdown. Click OK and the link to the file will be added to your text.

To link to an external website, type the address of the website where it says Link.  Alternately, you can just type a web address straight into the text editor (for example, http://digication.com) and it'll turn into a link.