I can't get it to arm. The ESC I am using is a Hornet 60a.
Have you looked at the Hornet 60a manual to see how to arm it?
FLY YOUR HORNET ESC
Your HORNET ESC comes ready to fly with Lithium Polymer type batteries. No settings changes are needed. To fly, turn your transmitter on before connecting your HORNET ESC to the motor battery.
When you connect the motor battery to the HORNET ESC, you will hear an initialization tone from the motor and a series of beeps telling you how many Lithium Polymer cells in series are in your battery pack, and the servos will be active. Make sure that the number of beeps matches the number of cells in your pack. If it doesn't, make sure your pack is fully charged.
The HORNET ESC will not arm unless you move your throttle stick to the lowest position. When the HORNET ESC receives the low throttle command, it will arm and play a double tone through the motor, indicating it is armed and now ready to run. If this is the first time you are running the controller, advance the throttle slowly to ensure that the rotor or propeller rotation is correct. If it isn't, swap any two of the motor side connections and it will then spin properly.
So use .write(0); or .writeMilliseconds(1000); to "move your throttle stick to the lowest position" and put in a delay long enough for the ESC to arm.