In the Mocksup editor you'll see an "Add a link" button.
Clicking on that button will allow you to create a clickable link by clicking and dragging anywhere on your mockup to create a rectangle link. When you're done drawing, a window will open asking you to "Select the mockup to link to".
Once you've chosen the mockup you want to link to and clicked the "Create new link" button, your link will be created.
Once a link has been created, it can be dragged to a new location, resized and edited or deleted via the toolbar that pops up when you hover over the link.