Hi,
The Valid User Code checksum is usually inserted into the binary by the debug probe. This could be the ULINK2 if you are using something like MDK+ULINK2, or the mbed interface if you're using the mbed Microcontroller.
As it happens the on-line compiler automatically inserts the checksum when compiling for the LPC11U24. This is so that if you use the on chip mass storage boot loader, you have a valid binary.
I am not surprised that the HEX file you generate from MDK doesn't have a Valid User Code Checksum. However, I am surprised that whatever you are using to load the hex file into the LCP11U24 isn't inserting it for you. What are you using to load the hex file?
If it helps, I wrote this program to patch binaries.. it runs on an mbed, and patches files stored in the local file system.
http://mbed.org/users/chris/code/BinaryPatcher/
Thanks,
Chris
The exported Keil project is not creating any Valid User Code checksum. For this reason the hex file created after compile is not running on a custom made LPC11U24 board. If I will program the board via SWD then the program is running (because the debugger is creating the correct checksum)!