The last step in configuring your site to work with PayPal™ or Square™ will add the On-Line Payments selection to everyone's My Stuff menu.
Go to Site Configuration → Security Configuration → Assign Tasks To Roles to see a page like the one shown below.
Scroll down this page until you see the entry for Make On-Line Payments. Click the button Assign To Roles to update this task, as shown below.
Check all of the boxes, then Save & Exit.
You should now see On-Line Payments on your My Stuff menu.
You are ready to use PayPal™ or Square™.