By default, it is not possible to add a store locator to your online store. Use the left menu of the back office to navigate to SYSTEM > App > App store. Here you will find an app called StoreMapper store locator especially designed to add a store locator. Note: this app is not free.
Do you prefer adding your own store locator. Paste its code in the HTML of the page in question or in the Template Editor. Note: Only proceed if you possess sufficient knowledge of HTML and CSS.