I want to use FreeRTOS WIZnet application on a LPC2106 with the KEIL uVision3 tools. Can you tell me how I can switch from GCC to the Keil tools?

The easiest way of achieving this would be to start with the Keil demo application from the FreeRTOS download, remove the demo application files from the project, change the target to LPC2106, then copy the WizNET source code into the project.  You will have to change the ISR syntax to that as demonstrated by the Keil demo (look at the Keil UART driver to see the syntax required). Make sure the startup code you use leaves the processor in Supervisor mode when main() is called!