Specialized Armadillo tires have relatively stiff sidewalls and, consequently, aren't the easiest tires to install. Try this:
1. With the tire completely off of the rim, install the inner tube in the tire.
2. Stick the tube valve stem through the rim and slip one tire bead over the rim.
3. STARTING OPPOSITE THE VALVE STEM start working the second tire bead onto the rim. Work both sides evenly.
4. When it starts to get hard, hold the wheel/tire assembly vertically in front of you. Place the unseated portion of the tire with the valve stem on the ground. Pinch both tire beads in toward the center of the rim all of the way around. Now grasp the tire at the 9:00 and 3:00 positions with both hands and try to push it directly down into the ground. What you are trying to do is to accumulate all of the slack at the uninstalled portion of the tire. Starting opposite the valve stem is important because that portion of the tube is a little thicker and uses up some of the slack that you need.
5. Most times that will give you enough slack so that you cah push the last portion of the tire on with your thumbs.