BrainTree Payment Gateway

Question

I have a link like 

https://www.braintreepayments.com/docs/dotnet/guide/overview

 

but i donot how to proceed .Any way i followed but i am getting error

https://www.braintreepayments.com/docs/dotnet/guide/secure_credit_card_storage  while following this link 

the error is like  Braintree.Exceptions.AuthenticationException 

 

Any one could you please explain how to do that ?I am trying this one from since last three days.Please tell me its very urgent