To search for a contact in your address book, simply, click on "Contacts" on your business dashboard as seen below.
Note, that you have 2 options. The first option as can be seen below is by using the search bar.
For the second option, click on the ABC box icon as seen below. Next, the alphabets a-z will appear on the right side of the screen.
Click on the first alphabet of the contact you're searching for and scroll through the search results for the contact.