The Nissan I-Key had the benefit of never having to use a conventional style key when locking, unlocking, or operating your Nissan. There is a small lithium battery that it located in the I-Key that should be replaced annually on both remotes. The I-Key system uses the remote as a transmitter to your Nissan which has several receive points located in various places inside the vehicle so it can determine if you are in the vehicle or outside of the vehicle. The I-Key is always transmitting a signal, even if you are not pressing any buttons (which is why the batteries should be replaced annually).
If the remote battery is dead, there is a metal key stowed inside the I-Key that can be released by sliding the lock that is located on the back of the I-Key body. This key is only used to unlock the drivers side door. After it is unlocked it should be stowed properly (some early model I-Key vehicles would use this key to start the vehicle). Once the remote is put back together, step on the brake pedal and use the I-Key to press the Start Button. If you have one of the earlier model vehicle there may be a dock port on the left of the steering column lower dash panel where you would insert the I-Key assembly into the port with the key ring eyelet facing out.
If you have not had the remote batteries replaced in a year or so you should consider replacing them soon. A low battery can result in sporadic operation and loss of range. The parts department can replace the batteries while you wait.