Try moving Shipdata.txt from your Starlancer directory (assuming it's there - or in one of the sub-folders) into the same directory you are running SLedit out of. Alternatively you may need to make sure you are running the most current version of SLedit (there are a number of patches for it you should download) and that you have unzipped/installed all the files associated with the program.

I'm tired, just woke up, and on my way to work, so if this doesn't help let me know and tomorrow when I have some more time I will actually look at the program and try to throw you a better idea of what's going wrong.

My apologies if this doesn't help at all.

Make sure you didn't extract SL Edit to the Starlancer directory. It won't work that way. If you did, uninstall SL and reinstall, and Install SL Edit to its own directory.