if you want to use mercadopago in Nexelit cms, At first you need to get your client id and client secrets , you have to setup your mercadopago gateway settings. Go to “General settings>Payment Gateway Settings > mercadopago settings”.
Credentials are unique passwords that identify an integration in your account. They allow to capture payments in online stores and other applications securely. They can be found on the Dashboard or in your Mercado Pago account by accessing Your Business > Settings > Management and Administration > Credentials.
for more info please visit
https://www.mercadopago.com.br/developers/en/guides/resources/credentials