The point that you're probably missing is that you don't do anything with the file you downloaded (except, perhaps, delete it) but you install DeVeDe using the Sabayon Package Manager. For this you can use Spritz but the fastest way is to use a terminal (how you find that depends on whether you're using Gnome or KDE - or one of the other GUIs - in KDE you'll find it in "System" and it's called Konsole).
Once you've opened it type in "su" (without the quote marks) and press enter. Then enter your root password and press enter again.
Then type in "equo update" (again without the quote marks) and press enter.
When it has finished updating, type in "equo install entropy" (without the quotes) and press enter.
When that has finished, type in "equo install devede" (without the quotes) and press enter.
When that has finished you should have DeVeDe installed.
You will probably find it useful to have a look at this page too:http://wiki.sabayonlinux.org/index.php?title=En:Entropy