Visit https://jamworks.com/ and click on 'Sign up'. All you need is your email address and your name to create an account. All other fields are optional.
Once you've completed the registration process, click on 'Download app' in the menu.
Then click the 'Download for Mac' button. Select the correct option for your machine.
Once downloaded, you will see the Jamworks file in your downloads folder. Double click to open the file and an installation window will appear.
Drag the Jamworks icon into your applications folder and follow the installation steps.
That's it, installation done. The first time you open Jamworks you'll just need to log in with the details that you used to register.