Venmo is a payment alternative offered through PayPal. If your site offers PayPal, it will also include an option for your customers to choose Venmo for their payment.
Venmo is offered as a choice on the checkout page.
Your customer should see the Venmo option when checking out of your store, unless they are located in a country that does not allow the use of Venmo.
To add the Venmo option, simply add PayPal as a payment option for your site, as outlined in THIS ARTICLE.