1. Go to DataStage Administrator->Projects->Properties->Environment->UserDefined. Here you can see a grid where you can enter your parameter name and the corresponding the path of the file.
2. Go to the stage Tab of the job select the NLS tab click on the Use Job Parameter and select the parameter name which you have given in the above. The selected parameter name appears in the text box beside the Use Job Parameter button. Copy the parameter name from the text box and use it in your job. Keep the project default in the text box.