QUOTE(twistedsymphony @ Apr 13 2007, 07:08 PM)
After you finish all of the main storyline goals you have to go back to see Isabella in the hideout before you head to the heli-pad. She doesn't call you and no one ever tells you to see here, but you have to see her before going to the helipad or you wont get the True ending.
It took me a couple of plays through to figure it out, the only reason I ever did was I got board and happened to go up there.
I'm not sure if there is a certain time or not but I'd check there regularly after you finish all of the main goals until she appears.