I saw your question was answered in the main sticky, but I figured I'd answer here as well for anyone who just sees this thread.
You can go to Guest Relations during the day and show your ticket to qualify for the Stay and Scream pricing for
HHN. You may want to make this one of your first stops for the day since the lines can get longer later in the day.
Since you have a park-hopper, If you want to take advantage of the Stay And Scream holding area which will give you a headstart from the general crowds, You will want to make sure you are inside Universal Studios by 4:30 and making your way to the holding area by Finnegans (The alley across from Mummy). I've seen recommendations of making sure you are in the park by 4pm just to avoid any unforseen delays keeping you from getting to the holding pen.
Universal Studios closes at 5pm. If you are not inside the park already and waiting at the S&S holding area, then you will need to wait outside the park for the 6:30 gate opening for everyone else. (S&S guests will be released earlier giving them a head start on the houses)
