Checking out the source code, installing the database and adding the demo data may be difficult if you are not an experienced developer with some free time.
The alternative is: download the Cinnamon-Server-VM
The username is 'tex', with password 'tex'. Once unpacked, it will require about 11 GByte of your hard drive. You will need either VMware workstation (commercial) or VMware Player (free) to run the VM.
The Cinnamon developer VM contains the source code for the Cinnamon 3 server, including the dependencies (cinnamon-db and cinnamon-humulus) as well as a PostgreSQL server with a demo repository. So, you just can start up Cinnamon from the command line and open the browser to start exploring Cinnamon.
You can even connect with the desktop client from a Windows machine, for example if you wish to debug the code.
Also included is the Spring Groovy Grails Tool Suite, which is a free integrated development environment.