How to control a robot wirelessly using xbee modules?
First you need to write a program that reads input from the PCs keyboard and sends commands to a serial xbee connection. Then you need software on the robot arduino that reads the commands from the serial connection and does what needs to be done. 

The definition of this protocol (the data sent over the serial connection) is entirely up to you. For example an "F" coming from the serial connection could mean that the robot should move forward, "S100" means "set speed to 100", "H1" means "turn on headlights" and so on. You can use linebreak or space to separate different commands.

This tutorial should get you started: http://tutorial.cytron.com.my/2013/01/14/remote-mobile-robot-with-xbee-wifi/