If you are able to connect your phone to the computer via usb follow these steps-

Download and install the official Nokia flashing software from the link below. Go to file menu>manage connections. If you see USB already in the list, then exit this dialog box else add USB by clicking on New and select USB. Connect you N900 to the computer. Now in the Testing menu select Factory Reset. Using this you can reset the security code.

If you cannot connect using the USB, flash you N900 using the Maemo Flasher. Just flash the rootfs. Now use phoenix to reset security code.