Assuming your controller has enough buttons to support all of the N64 buttons, all of those buttons are available by default if you configured all of the buttons on your controller during the initial gamepad detection. If you need to reconfigure it I recommend resetting your controller configuration in the Retropie system menu->Retropie Setup so that it prompts you again.
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first