By default the links you create on mockups have a subtle yellow background color. But, if you're using Mocksup for user testing and would rather the user find where they should click on their own, you can easily turn "link highlighting" off.
First, on your project's thumbnail page, click on 'Project Settings'.
From there click on the subtab 'Permissions & other settings'. There you'll see the on and off radio button for "Link highlighting".
Save your changes and your links will no longer be highlighted yellow.