Note: If you have not signed up with a credit card processing company, the "Rent Unit" button will not show up online even if you follow these steps.
1. Navigate to your Website Tab and click Pages.
2. Click edit next to Rent Storage. (This might say Rent Storage Units/ your city and state).
3. Scroll to the bottom of the page and under Current Widgets, click Edit under Available unit types.
4. Check the boxes for what you want customer to be able to do online. 
5. Click Update Widget.