From an entry form or the aircraft page, open the N-number picker and type the registration. The search runs against FAA aircraft registration data, so a valid US tail returns make, model, serial, category, class and engine type.

Pick the result and the aircraft profile is created and attached to your account. From then on it appears in your picker instantly.

Foreign or unregistered aircraft: add them manually. Enter the registration exactly as it appears on the aircraft, then fill in make, model, category and class yourself.

Simulators and training devices: create a profile for the device and log its time in the simulated instrument and ground-training fields rather than as aircraft flight time.