Retrieving datafied values

Hi, I'm (still) new to K2. I just want to retrieve a datafield value and do something based on the value returned.
Am I on the right path? When this code is executed, the error is that the user does not have permission to open a process instance. Thanks in advance.
SourceCode.K2ROM.Connection k2Conn = new SourceCode.K2ROM.Connection();
k2Conn.Open("myServer");

int processInstanceId = //the process instance id;
SourceCode.K2ROM.ProcessInstance pi = k2Conn.OpenProcessInstance(processInstanceId);

foreach (SourceCode.K2ROM.DataField df in pi.DataFields)
{
if (df.Name == "EvalStd1Subjective" && df.Value.ToString()==string.Empty)
{
//do something
}


}