When you downloaded the .pando file from your email, where did you save it? Search your hard drive for the .pando file using Spotlight. When you find the file, select it and drag it into the Pando main window. It should now show up in the Received list with a download button. If the download does not begin automatically, click Download and down it comes. Once the download is complete you can click Open.
Tip: When you save a .pando file from your email, put it in the same folder in which you save your Pando packages.