1: Select LINK as a plug type.
2: Configure your cart by adding one or more products.
3. Once you have configured your cart with your products and preferred payment gateway, copy the system generated LINK (URL).
4. Go to your Facebook page and edit the Call-to-Action button.
5. Paste the LINK (URL) you just generated into the Button editor of your Facebook page. Make sure to select "Make a purchase or donation" as the action for your button.
6. Hit "Save changes" and you are done.
...not to sound lazy, but it's pretty self-explanatory :)