In the final version of V2, will the API signature method be available for Paypal ? 
I don't have see any way to get certificate in my paypal sandbox account ?
will that be available in open source version ?

As long as API certificate is still supported there is not enough reason for us to provide the second option. Check the following link for the detail.

 https://www.paypal.com/IntegrationCenter/ic_shoppingcartresources.html

Step 3: Setup API Access

First-party authentication: Download API certificate

1. Log in to your PayPal account and click the Profile subtab.
2. Click the API Access link under the Account Information column.
3. Click the Request API Credential link.
4. Select the type of credential.
5. If you selected API SSL client-side certificate, click Download Certificate on the Download or Remove API Certificate page. You will be prompted to download a file called cert_key_pem.txt. This file is your API Certificate. Be sure to write down the name and location of the certificate.

Luke :

I can't find the same kind of scenario for Sandbox account, in developer site of paypal.
All I have is a link for API signature. Can you point me to the right direction ?

You'd better ask PayPal.

After talking to paypal, I found out what was happening. When I first requested API, I had both choices ( certificates or Signature), but I never looked carefully and chose the default API signature.
But, later in my account I couldn't find any way to request certificate anymore, as  paypal doesn't allow you to have both. Only one or the other.  Thanks for your suggestion t But, I'll still hope in future release of DSC,  API signature will be used as it looks like paypal is also prefering Signature to certificate these days.

Thank you very much for posting back the solution. We will consider to implement API Signature in the future. But it's not the high priority currently.