Download the Day Planner 0.11 Debian 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:
gdebi /path/to/dayplanner_0.11-1_all.deb in a terminal as root
This DEB can be used on any architecture (i386, ppc, x86_64, ia64, SPARC and so on) and should work on Debian 6.0 (squeeze) or later.