need help streaming data with UNO and SEEED GSM/GPRS V2.0

configuration:
-iOS mavericks
-arduino uno
-SEEED GSM/GPRS V2.0
-unlocked and verified AT&T SIM card with data only (No SMS or Voice Calls)

How do I stream voltage readings from an analog sensor onto the web?

I'm using m2x.att.com as my API, but I'm open and interested in other options--perhaps something with a better support community.

This is the code I'm using:

#include "SerialModem.h"
#include "jsonlite.h"
#include "M2XStreamClient.h"

SerialModemClient client;
char m2xFeedId[] = "df60542f9f2ee9c1fed588320363b823";    // Feed you want to post to
char m2xKey[] = "30700b29c45af8922ec95e659e5da7a0";    // Your M2X access key
M2XStreamClient m2xClient(&client, m2xKey);

const int SENSOR = 0; // selects the input pin for the thermistor

int val = 0; //variable used to store the value coming from the sensor

void setup() {

  // Note: Analogue pins are always and 
  // automatically set as inputs

  Serial.begin(115200); // in tutorial it was 9600
  while (!Serial);

  SerialModem.setDriver(DRIVER_MTSMC_H5);
  SerialModem.setSerial(&Serial, 115200);
  while (!SerialModem.ready()) {
    Serial.println("waiting for modem ready..");
    delay(100);
  }

  while (SerialModem.getNetworkStatus() < NETWORK_STATUS_ROAMING) {
    Serial.println("waiting for network registration");
    delay(2000);
  }

  while (!SerialModem.setAPN("wap.cingular")) {
    Serial.println("setting APN");
    delay(2000);
  }
}

void loop() {

  val = analogRead(SENSOR); //read the value from the sensor

  Serial.println(val); // print the value to the serial port

  delay(1000); // wait 100 ms between each send

}

I also tried using the following code to test my modem:

/*

 This example test if your modem is working correctly.

 Circuit:
 * GSM shield attached (using digital pins 2, 3, and 7)

 Created 12 Jun 2012
 by David del Peral
 modified 21 Nov 2012
 by Tom Igoe
 */

// libraries
#include <GSM.h>

// modem verification object
GSMModem modem;

// IMEI variable
String IMEI = "";

void setup()
{
  // initialize serial communications
  Serial.begin(9600);

  // start modem test (reset and check response)
  Serial.print("Starting modem test...");
  if(modem.begin()) 
    Serial.println("modem.begin() succeeded");
  else
    Serial.println("ERROR, no modem answer.");
}

void loop()
{
  // get modem IMEI
  Serial.print("Checking IMEI...");
  IMEI = modem.getIMEI();

  // check IMEI response
  if(IMEI != NULL)
  {
    // show IMEI in serial monitor
    Serial.println("Modem's IMEI: " + IMEI);
    // reset modem to check booting:
    Serial.print("Reseting modem...");
    modem.begin();
    // get and check IMEI one more time
    if(modem.getIMEI() != NULL)
    {
      Serial.println("Modem is functoning properly");
    }
    else
    {
      Serial.println("Error: getIMEI() failed after modem.begin()");
    }
  }
  else
  {
    Serial.println("Error: Could not get IMEI");
  }
  // do nothing:
  while(true);
}

and I got "Error: Could not get IMEI"

I also gave up on m2x. I've tried xively as well, but that is not going well either. I've got the same setup as you. What I do know though, is that the .setDriver method in your M2x code refers to a model of gprs shield that is different from yours.