How to connect client applications to SQL SERVER 2012 cluster?
Neither of those should work. If it's a named instance, it should be:

DATA SOURCE=ClusterName\SQLInstanceName;


If it's a default instance, just:

DATA SOURCE=ClusterName;


You are always connecting to an instance of SQL Server, whether it is clustered or not. The cluster will know which node in the cluster is currently hosting the instance, and this should not really be a concern of yours, at least in terms of the connection string - that's kind of the whole point, a failover to a different node in the cluster should not require your application to change anything.