This error can arise because XDS has been run from within it’s own directory, on a machine where the XDS directory is not in the PATH variable.
You can check if the path is broken (did you move the directory containing XDS?) in the Terminal
(you’d better put capitals)
On Snow Leopard and beyond, you must add the XDS directory to your path by editing the .profile file in your home directory. I’ve written about how to edit these files from the terminal before (but any text editor will do if you are cunning with filenames).
Briefly, according to the XDS dowload page, you will do well do add the following lines to .profile, assuming that you put XDS into “applications” and you are running Mac OSX:
export PATH=/Applications/XDS_OSX-Intel/:$PATH export KMP_STACKSIZE=8m
You’ll now be able to run XDS (including XDS_par) from any directory, and XDS itself shouldn’t have any problem launching it’s own components.