As the release of MCPE 0.7.4, PocketMine Realms has been closed by petition of Mojang. More details on the PocketMine Realms homepage
PocketMine Realms was a hack of Mojang's Realm service, but once they added the ability to add external servers in MCPE, Johan requested that they remove the PocketMine Realms service, their servers, and any code that they used. As for March 1st, Mojang has closed their Realms service until a later time this year, but external servers will still work. Besides, if Mojang's Realms service is offline, PocketMine Realms wouldn't work either (if it still existed, that is).