From the main screen of the app tap the 3 bar menu:

Upon tapping this the following screen will appear:

Tap on "Advanced" then "Customers" to bring up the following screen:

Tap the plus sign at the top right to add a new customer.

In the Full Name field enter the customer number. In the Company field enter the store name. 

The rest of the information is not needed but you can add it if you wish.

After you have entered the information for the customer just click "Add" at the top right of the screen. This customer is now in the systems and should appear for leasing out assets.