Download the Day Planner 0.11 Ubuntu package.
When downloading, select "Open with: GDebi" instead of saving it.
If you really want to save it, you can simply click on the downloaded file in your file browser to install it.
Power users can also install it using the commandline gdebi tool by typing:
sudo gdebi /path/to/dayplanner_0.11-1_all.deb in a terminal
This DEB can be used on any architecture (i386, ppc, x86_64, ia64, SPARC and so on) and should work on Ubuntu 10.04 or later.