How to know the direction that the Robot is pointing to from pose?

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I think you might have mixed up some words (and since english isn't my native language, the following might not be 100% correct):

A quaternion always has 4 values. Since your "pose" has 3, it either is the position in xyz and you dereferenced the values wrong. Or it is the orientation in euler values, most likely in r(oll) p(itch) y(aw), which basicly is the value you are looking for in radians.

As I said, at this point I am not to certain if "pose" is either another word for "orientation" or for "position". But those values are definetly not a quaternion.

Here's a sample nav_msgs/Odometry message, the first part of which is a geometry_msgs/Pose message:

header: 
  seq:        239
  stamp: 
    secs: 1408542253
    nsecs:  730130796
  frame_id: odom
child_frame_id: base_link
pose: 
  pose: 
    position: 
      x: 19.4294389
      y: -1.4772684
      z: -0.6923008
    orientation: 
      x: 0.01773404
      y: 0.01089619
      z: -0.0490339
      w: 0.99858021
  covariance: [0.00145422, 0.00000001, 0.00000018, -0.0000000, 0.00000000, 0.00000000, 0.00000001, 0.00145451, -0.0000003, -0.0000000, -0.0000000, 0.00000000, 0.00000018, -0.0000003, 0.01903145, 0.00000000, -0.0000000, 0.00000000, -0.0000000, -0.0000000, 0.00000000, 0.00000099, 0.00000000, 0.00000000, 0.00000000, -0.0000000, -0.0000000, 0.00000000, 0.00000099, -0.0000000, 0.00000000, 0.00000000, 0.00000000, 0.00000000, -0.0000000, 0.00000099]

The part you need is the "orientation." It is represented as a quaternion instead of Euler angles. To get the Euler angles, you need to carry out a conversion. If you want to do it with C++, you can do something like this:

tf::Quaterion quat;
tf::quaternionMsgToTF(msg->pose.pose.orientation, orientation);
tf::Matrix3x3 orTmp(quat);
orTmp.getRPY(roll, pitch, yaw);

If you're doing it in Excel, you'll need to carry out the conversion yourself. If you're simply after the robot's heading, you want the "yaw" value.