Well, I try to start the quest to save President Kimball. This quest begins when you talk with Ranger Grant. However I am not found Grant. I searching for him in: Visitor Center both floors, Pawer Plant level 1 and 2, in the Tower, I climb the stairs of Visitor Center, I try to talk with Colonel Moore.

Could anyone help me?, I forgot something ?, or better forget this quest and finish the game.

Quest marker?

This part is the most strange, When I talk to Yes Man for the first time he said that I should save Kimball. However, this quest dont showed up. And if I talk to Yes Man for the second time (I killed Mr House) the quest failed.
Now do you understand because I thing that is a bug?!

P.S: And of course, dont show any quest marker.

It sounds as though Kimball came and died before you could start the quest.
Grant should be in the lower lobby of the visitor centre.

1. You can't get the quest from the 1st talk with Yes Man :confused:
2. Of course, it shouldn't
3. As in when hes in the mainframe?

:shrug:

4. http://fallout.wikia.com/wiki/You%27ll_Know_It_When_It_Happens#Bugs, maybe that will be more helpful.

1st floor of visitor center

-Cheers