How to display Renew button? How to add link to renew in auto email?

I want to add a link to the email sent to the member before expiration that allows them to renew their membership. I am struggling with getting the renew button to appear and don't know where to point the link (in the email) to.
I read the following in the forums: "If you are using a single payment gateway (Authorize.net or PayPal Single Payments), then you should set the number of days before expiry that the renewal form is displayed on the Account page"...this must be to some other version because I can't find it. I am using PayPal Single Payments.
Please advise.