In Store -> Payment -> PayPal Standard, you could enable PayPal Standard payment module. Before you enable PayPal Standard, make sure you have a valid PayPal account. To enable "PayPal Standard", click "Enabled" checkbox. Enter other neccessary fields that are required to complete PayPal Standard payment module. Here are some field explanations.

• Use Sandbox: when you enable this, all the transactions are redirected to PayPal Sandbox which is a testing environment. If you want to test PayPal Standard model, you could enable this. Be sure to turn it off before go live.
• Business Email: your PayPal account login email.
• PDT Identity Token: login to your PayPal account. In Tab "My Account" -> Profile -> Selling Preferences -> Website Payment Preferences, turn on "Auto Return" and set "Return URL" to https://[your store URL]/Store/PaypalPDTHandler.aspx . Also you need to turn on "Payment Data Transfer" and then click "Save". When you click Save and save your PDT preferences, a message appears at the top of the page indicating that you have successfully saved your preferences. Your PDT Identity Token also appears in this message.

Do not forget to click "Save" button when you are done.

 You are not done yet. You need to turn on Instant Payment Notification in PayPal account. Log in to your PayPal account and go to Tab "My Account" -> Profile -> Selling Preferences -> Instant Payment Notification Preferences. Check the checkbox to activate Instant Payment Notification and put https://[your store URL]/Store/PaypalIPNHandler.aspx to the notification URL text box.

*[your store URL] is the web url of your online web store. You can find it under Site -> General -> Address -> Site URL