The voltdb documentation discusses these issues you can find it in the section on designing stored procedures, here: http://voltdb.com/docs/UsingVoltDB/DesignProc.php
VoltQueueSQL and VoltExecuteSQL are VoltDB stored procedure semantics, you don't use these in your client program, I think you are confusing these. Your client program invokes the stored procedure (using callProcedure), in your case its called 'insert' once it has an open connection estabished with the cluster. The stored procedure 'insert' runs on the cluster (server side) and executes queries using VoltQueueSQL and VoltExecuteSQL.
From the manual, some other key points that relate to your questions are:
#5. To perform database queries, you queue SQL statements (specifying both the SQL statement and the variables to use) using the voltQueueSQL method.
#6. Once you queue all of the SQL statements you want to perform, use voltExecuteSQL to execute the statements in the queue.
#7. Each statement returns its results in a VoltTable structure. Because the queue can contain multiple queries, voltExecuteSQL returns an array of VoltTable structures, one array element for each query.