First, win the circuit trophy normally or by entering this code:
8DP5KG5L4G59P
G92WVCQY0DRDQ
With the trophy won, enter 'One Race' mode and do the following:
1)At the 'Select Car' screen, hold Left C, press Z, release both, tap Left on the D-Pad, then press B three times to return to the main menu.

2)Choose 'Setup', and at the setup screen, hold Up C, press Z release both, tap Up on the D-Pad, then press B to return to the main menu.

3)Choose 'One Race', and at the 'Select Track' screen, hold Left C, press Z, release both, and tap Right on the D-Pad, then press A to advance to the 'Car Select' screen.

4)At the 'Select Car' screen, hold Down C, press Z, release both, tap Down on the D-Pad, then press the L button, then press the R button. You will hear a signal to confirm the code. Now press B to go back to the 'Select Track' screen, and you will find that the secret Track 7 is now unlocked.