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 "Activate SM 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 and click OK. (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 "Currently Playing" field). You can assign music to your sub-accounts by using the assign music button under the Playback section.