He pleaded guilty to entering a Park Ave Waitarere Beach without authority and intent to commit an imprisionable offence in that building.
McIsaac was remanded in custody until July.
Justice Joe Williams praised Alex's family for the courage they had shown: "I understand this is a very difficult time for you...I salute your courage."
Alex's mother cried throughout the proceeding.
In a statement Police noted the guilty pleas entered by McIsaac in court today, but said they would be making no comment until the conclusion of the court process at sentencing.
"Police would however like to reiterate the wishes of Alex Fisher's family that they are not to be approached by media. They would like their privacy to be respected while they continue to grieve for the loss of Alex."