Crash Reports are in the folder /Users/<you>/Library/Logs/DiagnosticReports.
You can get to them by opening Finder, and choosing Go to Folder from the Go menu.

Enter in the path above. Note the use of the tilde (~) character at the front. This means "starting from your home folder".

You'll now see a bunch of files.
You want only those beginning with a certain name. i.e: iShowU HD (it depends upon what you've been asked to collect). Here I've created three crash reports (iShowU would never crash would it? :). I've then highlighted the three reports and clicked the right button. I'm about to compress them into a single archive for easy transfer.

You'll end up with an archive. That's what we're after.

Enter in