dimension question

[POP]

Hi all .... just trying to manually dimension exterior wall to centerline of windows and doors.......I set up in dim to go to center of whatever .....when not in dim mode ....when selecting just a door the dimesions I want show up automatic, but only when the door is highlighted....once I try to enter the dim to stay, they just disappear.....??? I just want to manually dim end of wall to center of door or window.....any help would be great....thanks POP