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 ( 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.