You can't open it.
Here's what to do:
1) Close the app
2) Use spotlight to locate to plist file
3) Right click in spotlight on the file, and select "Reveal in Finder"
4) When the finder window opens showing you the plist file, drag the file to the desktop
5) Re-open the app, see what happens
6) If bad things happen, move the plist file from the desktop back to where you took it from.