Have you gotten to winter yet? If not, you will not see that tab because you haven't had the opportunity to get the "magnifying glass" yet.
If it is winter, to get that unlocked, you must enter the bus stop from the farm between 6am and 4pm. That will start the mini-quest that unlocks the "magnifying glass" for you. The "magnifying glass" is what unlocks the ability to find secret notes and the tab for them in the menu.