I came about this question and found it useful, but no answers, so after figuring it out myself I decided I should help the next poor sucker to find themselves in this position too.

I followed the comments and installed kdl_parser. I followed the instructions to convert my .xacro into an actual urdf file:

rosrun xacro xacro.py `rospack find pr2_description`/robots/pr2.urdf.xacro > pr2.urdf

From there I installed orocos_kdl by following these instructions. I will admit that i'm fairly new to ROS/Catkin, so to figure out how to actually add these packages to my own wasn't very obvious. I modified the CMakeLists.txt and added the packages to the find_package portion of the CMake, as well as adding to the package.xml as the documentation instructs.

I was able to confirm I got everything working by building a simple executable to test to make sure that I was able to make a tree from my urdf file:

#include <kdl_parser/kdl_parser.hpp>
#include <kdl/tree.hpp>
#include <ros/console.h>
#include <iostream>

int main(){

  KDL::Tree my_tree;
  if(!kdl_parser::treeFromFile("src/robot_arm/src/urdf/robot_arm.urdf", my_tree)){
    ROS_ERROR("Failed");
  }
  std::cout << "Worked!" << std::endl;

}