Where do I put base_footprint?

edit

So I have a urdf for my robot, with the tree root as base_link.

However, I am using robot_pose_ekf, which has a hard-coded transform for base_footprint. Assuming a simple square base_footprint, should I make it the root:

base_footprint
  base_link ... rest of model

EDIT:

After adding a base_footprint link and joint to base_link:

robot name is: Thumperbot_Simplistic
---------- Successfully Parsed XML ---------------
root Link: base_footprint has 1 child(ren)
    child(1):  base_link

I still get the following error:

Node: /robot_pose_ekf
Time: 12:35:35.441279569 (2014-09-16)
Severity: Debug
Published Topics: /robot_pose_ekf/odom_combined, /rosout, /tf

Could not transform imu message from base_link to base_footprint. Imu will not be activated yet.

Location:
/tmp/buildd/ros-indigo-robot-pose-ekf-1.11.11-0trusty-20140805-0105/src/odom_estimation_node.cpp:OdomEstimationNode::imuCallback:234

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Perhaps it is not receiving my urdf that have in the launchfile? Here is the launchfile:

<launch>
  <param name="robot_description" textfile="$(find thumperbot_description)/urdf/thumperbot_description.urdf" />
  <node name="gps_conv" pkg="gps_common" type="utm_odometry_node">
    <remap from="odom" to="vo" />
    <remap from="fix" to="/gps/fix" />
    <param name="rot_covariance" value="99999" />
    <param name="frame_id" value="base_link" />
  </node>
  <node pkg="robot_pose_ekf" type="robot_pose_ekf" name="robot_pose_ekf">
    <rosparam>
      odom_used: false
      imu_used: true
      vo_used: true
      debug: true
      self_dignose: true
    </rosparam>
  </node>
  <include file="$(find piksi_driver)/launch/piksi_driver.launch" />
  <include file="$(find razor_imu)/launch/razor-pub.launch"/>
</launch>

Here razor_imu and piksi_driver are drivers for the razor imu sensor and the piksi gps sensor. Both sensors output their data correctly.