Installing the Client Audit module is exactly the same as installing any Form Tools module. First, download the zip or tar file from the Downloads tab above. Then, follow the instructions here.
The Client Audit module works by tracking changes to client accounts. As such, when it's first installed, the module automatically stores the current state - account, account settings and account permissions - in its custom logs. Because of this, if you have a large number of client accounts, it may take several seconds to configure the database.
After installing the module, select it. You'll be taken a page that will tell you there's been no activity on your client accounts. This is normal. At this point, the module is enabled and active: as soon as any changes occur to any client accounts, you will start seeing some logged items appear.