This tutorial should show you the steps necessary to control a servo with an Arduino or Arduino clone micro-controller with MyRobotLab.

Connect the Arduino to a computer which will be running MyRobotLab.  Load the ArduinoSerialBare.PDE on the Arduino through the Arduino IDE.  Start MyRobotLab.  Load an Arduino service with an appropriate name.  In this case I chose "arduino" (creative no?).