If you use the html mode of the editor, you can enter the link as this:
<a href="http://yourlink">your text</a>
If you use the visual editor, you should type your text, then drag your mouse to highlight the text you wanted linked. Then click the link button (looks like a chain) in the editor and a popup appears. Type your link into that to insert the link. If you do that and you get an error, please leave the error there and publish your post, then please PM me so I can check it from the back end and find out what's wrong. Thanks