Login to the backup portal from the link given when you created the account to you and and click on "Protected Items"

Click on Add to protected item.. and select the NAS device

Once the device is selected, now you can begin to create a backup set. Keep it as a Files and Folders backup but proceed to give it a name and click on Next >
Add a path to the backup via the "+" icon, for example \volume1, and then click on Schedule
Give the backup a schedule using the "+" icon, when you are finished press Save.
If you would like to test this works you can run a manual backup. Go back to your portal and select Devices, then Backup.
Here you will see all the information on your backup, you may need to use the dropdown sections if you have more than one Device and Protected Item setup. Ensure the information is correct and press Backup.