Which Nessus connector should I use to pull the vulnerabilities from cloud.tenable.com?

You can use the Nessus "Importer" connector, which will allow you to select one or more scans to pull in results for. 

Enter the port with domain port with the domain: cloud.tenable.com:443

Enter Access Key and Secret Key to connect. Use of Username/Password to connect has been deprecated by Tenable.