To add a Soundmachine hardware player to your account, click on the Users tab:

Find the account you want to add a hardware player to, and select "Add Device":

Find the activation code on the bottom of your player, or on the box your player was shipped in, then enter the code into the fields. (Please note that the activation code is also the MAC address of the player):

Your player will automatically resume the last playlist you were listening to (which can be identified in the "Scheduled" column). You can assign music to your sub-accounts by using the assign button next to each name.