I'm having a bit of a nightmare adding the assigned to box with popup to the mass update submenu. Is there any chance someone could shed some light on this for me?

I've tried changing the vardefs for assigned_user_id and assigned_user_name massupdate to true and this hasn't worked. Can anyone offer me an idiots guide to this? I've tried the wiki and I've been searching the forum and I can't find anything that works. I'm using Version 4.5.0b (Build 1144)

many thanks