Hi Brian, I have good news and bad news.
First the bad news: The dashboard doesn't provide a way to download a snapshot right now. This is just something that's not provided in OpenStack's implementation.
The good news is that you can download it using the command line client* for glance. Once you have that installed, you can perform the image-list subcommand to view the available images. You'll see the default server images as well as your snapshots. Grab the ID of your snapshot and then use the image-download sub-command. Here's the usage info:
$ glance help image-download
usage: glance image-download [--file <FILE>] [--progress] <IMAGE>
Download a specific image.
<IMAGE> Name or ID of image to download.
--file <FILE> Local file to save downloaded image data to. If this is not
specified the image data will be written to stdout.
--progress Show download progress bar.