First of all, congrats on starting with Max2Play 🙂
Now for your issue, you have multiple options to control the playback of your Squeezelite player. However, the player itself does not have any interface to control it. The control is handled entirely by the LMS. So you need to access the web interface of the LMS on your NAS. It also has a port similar to 9000, perhaps 9001 or 9002. But you can check it in the network overview of your PC or in the web interface of your NAS.
Another, more convenient way to control your playback is through one of the many great apps for Squeezebox. If you have an Android device, check out Squeezer. With iOS, we recommend iPeng and with Windows, you can with SoftSqueeze. These applications automatically servers and palyers in your network and let you control their playback.