Hi Mario,
as already written by PaulS please post code in your message using the code tags ( => "[ code]put your code here [/ code]" <= use without whitespaces between "code" and "[" and "]" to work)
Nevertheless I looked at your code in the picture and your problem is that you have not posted the variable "c" to PLX DAQ but the string "C" hardcoded.
Simply use
Serial.println( (String) "DATA,DATE,TIME," + c );
and it should work
Greetings!