InfoPath Dev

How do you get the form's Global Directory items to access the organization's Global Directory?

My experience with radio buttons is that they are single select. When I try to insert them, I cannot apply rules for single selection like I can for checkboxes. Say I want an A button and a B button and want the user to select only one or the other, how do I make it so with radio buttons?

How do I make a text box appear when a certain dropdown selection is made? When I try to create a rule I don't know how to call on that particular text box in the rule. Your help is much appreciated.

Is there a way to create a field using a substring which displays time entered by a user but without a default time?

Can anyone recommend a substring that allows for the creation of an InfoPath Time Picker without default?