Custom download button added into current wordpress theme.

Hi I'm looking to add a simple "download here" button to my current theme on wordpress in this manner, [login to view URL] My website is [login to view URL] so you can get a feel for how I want the download button to work. If you click on mixtapes on the top section of the header, and now you see I have a portfolio type of display where you can click the images to open up a lightbox. I want to be able to have a "download mixtape here" button that allows us to add links to specific downloads once the light box opens as described in the previous sketch. Thanks for you time, and looking forward to hearing back! - Mike