First double-click on the text box that you want to edit. Then click the 'Link' tab in the SiteMaker Editor to see the link options available. Select the piece of text you want to make a link by highlighting it with your mouse. Choose the link type by clicking on one of the link icons in the 'Link' tab (don't worry if your text deselects, SiteMaker remembers what was highlighted).
SiteMaker provides a number of different link types, e.g. webpage, email link, download file, page within your site, PayPal ecommerce link and login/join link. Each link type will require you to add specific information. Follow the on screen instructions once you have selected your link type, and then use the 'Apply' button to confirm. Remember to save your page once you have made changes.