Ok, so reading around a bit I am wondering if you have a ~/Desktop or similar directory. To find out:
This will list ALL files in any given directory, so make sure you are in your user's home directory. Then type that up there to see if there is a directory called something like that. There is supposed to be a file in there called Autostart, add the kstart entry to this file.
The other option is the actually edit xdm. We will hope you have an Autostart file first, then move deeper if we need to.