You can put your TI-Nspire handheld in 84 Mode by removing the standard TI-Nspire key pad (the release is located on the back of the handheld) and replacing it with the TI-84 Plus Keypad. The 84 keypad was included with TI-Nspire Clickpad handhelds sold in the US and Canada and available for free from TI for TI-Nspire Touchpad users upon request. http://education.ti.com/84keypad)
The TI-Nspire CAS handhelds (all models) and TI-Nspire CX handhelds do not support 84 Mode. These handhelds do not have a removable keypad and/or support the 84 Keypad.