PayFast Integration is not working

I am trying to get PayFast as a payment option on MarketPress. I followed the instructions at https://www.payfast.co.za/shopping-carts/market-press-ecommerce/ but no luck. When I copy the files over the plugin causes my website to crash and I have to uninstall it manually.

When I reinstalled the MarketPress plugin I noticed that PayFast.php is already in the payment gateways folder, but it is not available as an option in the store settings.