The correct way to install RainLender in Ubuntu 16.04 is as follows
.deb file from their download site labeled with (Debian/Ubuntu). I am here demonstrating from terminal for 64-bit architecture.
wget -c http://www.rainlendar.net/download/rainlendar2-pro_2.13.1.b147-1_amd64.deb
Change the link to pro or lite depending on your need in the command.
Once the download is complete, install it with
sudo dpkg -i rainlendar2-pro_2.13.1.b147-1_amd64.deb
Again change the filename accordingly. The terminal can autocomplete the filename for your if you press Tab key twice quickly.
Though this will install rainlender, but the software would stay in unconfigured, because some of it's dependency will not be satisfied. You may see the output like this after
Selecting previously unselected package rainlendar2-pro.
(Reading database ... 651477 files and directories currently installed.)
Preparing to unpack .../rainlendar2-pro_2.13.1.b147-1_amd64.deb ...
Unpacking rainlendar2-pro (2.13.1.b147-1) ...
dpkg: dependency problems prevent configuration of rainlendar2-pro:
rainlendar2-pro depends on libwebkitgtk-1.0-0; however:
Package libwebkitgtk-1.0-0 is not installed.
dpkg: error processing package rainlendar2-pro (--install):
dependency problems - leaving unconfigured
Processing triggers for bamfdaemon (0.5.3~bzr0+16.04.20160523-0ubuntu1) ...
Processing triggers for gnome-menus (3.13.3-6ubuntu3) ...
Processing triggers for desktop-file-utils (0.22-1ubuntu5) ...
Processing triggers for mime-support (3.59ubuntu1) ...
Errors were encountered while processing:
To fix this dependency problem, again type in terminal
sudo apt-get -f install
It will then download and install other packages to fullfill dependencies and you can then run it.
Here is the screenshot running RainLender in my Ubuntu (with KDE5)