An example of how to connect to the xively MQTT server using arduino. Based on the pubsub client by knolleary

xively mqtt arduino

/*
  Xively MQTT example 
 
 -Subscribes to Xively feed
 -publishes current value to serial monitor

Based on Basic MQTT Example by knolleary

By Calum Barnes, Xively (c) 2013

BSD 3 License

*/
#include <SPI.h>
#include <Ethernet.h>
#include <PubSubClient.h>

// Update these with values suitable for your network.
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
byte server[] = { 64, 94, 18, 120 };
byte ip[]     = { 192, 168, 1, 10 };
String stringone = "api.xively.com";


void callback(char* topic, byte* payload, unsigned int length) {
  Serial.println(topic);
  //convert byte to char
  payload[length] = '\0';
  String strPayload = String((char*)payload);
  
  Serial.println(strPayload);
  int valoc = strPayload.lastIndexOf(',');
  String val = strPayload.substring(valoc+1);
  Serial.println(val);
  
  
}

EthernetClient ethClient;
PubSubClient client(server, 1883, callback, ethClient);

void setup()
{
  Serial.begin(19200);
  Serial.println("==STARTING==");
  
  // start the Ethernet connection:
  if (Ethernet.begin(mac) == 0) {
    Serial.println("Failed to configure Ethernet using DHCP");
    // no point in carrying on, so do nothing forevermore:
    // try to congifure using IP address instead of DHCP:
    Ethernet.begin(mac, ip);
  }
  // give the Ethernet shield a second to initialize:
  delay(1000);
  Serial.println("connecting...");
  
  for (byte thisByte = 0; thisByte < 4; thisByte++) {
    // print the value of each byte of the IP address:
    Serial.print(Ethernet.localIP()[thisByte], DEC);
    Serial.print("."); 
  }
  
  //delay(500);
  boolean con = client.connect("arduinoMQTT", "XIVELY_API_KEY", "");
  while(con != 1){
    Serial.println("no con-while");
    con = client.connect("arduinoMQTT", "XIVELY_API_KEY", ""); 
  }
  //Serial.println(con);
  if(con){
    Serial.println("got con");
    //client.publish("outTopic","hello world");
    client.subscribe("/v2/feeds/FEED_ID.csv");
  }else Serial.println("no con");
  
}

void loop()
{
  client.loop();
}

Hello, do you send your MQTT PUBLISH and receive MQTT SUBCRIBE through HTTP or just TCP socket? Can you printf the datastream you send and receive? Thank you.