When I make a user to be the presenter, I want to ask him for mouse and keyboard control. How this can be done? I didn't find any button or menu to ask that.
There is not a button in the GoToMeeting Control Panel to request Keyboard and Mouse control, you would need to send a message through the chat or ask over the conference call.
Once they are presenting they would click on your name in the attendee list and give you mouse and keyboard control. Keep in mind that GoToMeeting does not run on a users computer with admin rights, so control is limited.