You can easily get the program in source form. First you'll have to get the sources (see below), then you'll have to install them. Regarding the installation:
More detailed installations instructions can be found in the sources.
Regarding how to get the sources, there are two ways to get the program in source form: the official source release and the CVS repository:
The latest source release of the OneFinger core and OneFinger applet are available here.
cvs -z3 -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/onefinger checkout onefinger
This will create a directory called onefinger in your current dir, and put the source code of the OneFinger core in it.
To download the OneFinger kicker applet:
cvs -z3 -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/onefinger checkout kicker_applet
This will create a directory called kicker_applet in your current dir, and put the source code of OneFinger kicker applet into it.
You can also browse the source code online with sourceforge: simply click here.