To do that go to Merchant tab >> Payment gateway.
Choose 'Braintree' from the drop down list
Go to your www.braintreegateway.com and login with your credentials.
In Braintree panel go to Account -> API Keys.
Copy "Merchant ID" to Gateway ID in My Hub.
Copy "Public key" and "Private key" to the proper fields.
Copy "CSE key" to CSE key in My Hub.
Copy your Merchant Account ID for the selected currency to "[currency] Account" in My Hub. To find your Merchant Account ID go to Settings -> Precessing, scroll down and you will see your Merchant Account IDs
More details about your merchant credentials you can find in the Braintree payments knowledge base.