Dianne,
The following should fix the problem:
1. Open terminal application
2. Assuming run the command, "chmod +x <xnd installation folder>/
xnd.app/Contents/MacOS/xnd", for instance if you extract xnd into
"xnd" folder on your desktop, the command will look like: ""chmod +x ~/
Desktop/xnd/
xnd.app/Contents/MacOS/xnd"
Sorry for this inconvenience.