Like LittleMaster says enter the ELAN IP of the server you want to use as the PD server, any of the SS will do depending on the number of extensions might depend where you put it. For example if you had several thousand users it would be best to have a PD server that doesn't do anything else (No TPS, VTRK, etc.), if you only have a few users you may just have one SS doing everything. Once you assign the server ELAN you need to restart the phones for it to show up.