From version 2.7.6 and onwards SocialAds adopted the Common Payments API to make it possible to integrate new payment methods faster and have a better tested payments interface that was tested across the community. This Documentation will give you a quick look at setting up payment methods in SocialAds.
Setting up the payment plugins involves two main steps
Enable and configure the Payment plugins you need to use
You can read the common documentation for this here. This is applicable for all extensions using Common payments API.
Once the plugins you need are configured as given above, you need to configure SocialAds. To do so go to Components > SocialAds. There go to settings. Under the Pricing tab, you should be able to choose the Payment plugins to use.