@loddem, your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with your project 
See About the Installation & Troubleshooting category.
You basically have 3 options to communicate. Serial, I2C and SPI; you can use those to data from the Uno to the Mega.
Do you have a link to the robot car that you bought or preferably the schematic of the shield.