Use the Pop-up Media option in the popup to point to the hyperlink field using an image and then the hyperlink will work.Showing images in pop-up windows:<a href="http://resources.arcgis.com/en/help/arcgisonline/index.html#/Showing_images_in_pop_up_windows/010q000000m8000000/" target="_blank">http://resources.arcgis.com/en/help/arcgisonline/index.html#/Showing_images_in_pop_up_windows/010q000000m8000000/</a>