Hi

First of all (after having copied the link), please click on your text-field (you must have written a text first or use a picture), mark it, then go to "characteristics" (I hope that this is the right expression, as I am using the German version of the Website Maker) on the left, click on "linking", choose "external link" and copy your link-details into the field that is opening (complete incl. http://www....). As soon as you have put in the link, your text will be underlined.
Do not forget to save the alterations! Links only work in the "preview"-mode or on the published website not on your edting-layer.
Cheers

Aussie-Elly