Installing the on-screen keyboard is not the solution to develop software or write long text documents on the Raspberry PI, but it is a must-to-have to make complete the user interaction options. Installing the keyboard is a very simple operation that can be done with a single command from the terminal:
sudo apt-get install matchbox-keyboard
just as it is explained in the product document page on Element14. The documentation says that the keyboard (after installation and reboot) is accessible on-screen from the Menu->Accessories->Keyboard. Doing several installations I have verified that the keyboard is not always listed as a new menu option; one of the reasons depends on the previous presence of a physical keyboard connected to the Raspberry or not. To be sure that the on-screen keyboard is added to the desktop menu after installation the better choice is to manage the Raspberry PI from remote via SSH.
The on-screen keyboard can be shown by your applications or bash scripts with the command