How do I use this tool?
 

please use the triangle/exclamation tool to report a post, your link took me to the thread, but not the post

if you are asking how to use the icon to report a post, just click on it.
type in the reason you are reporting it and send the report.
it will go to all the moderators.

I thought there was a way to indicate a specific post in a specific thread.

Say I wanted to say that in thread XYZ, post #78 had personal information in it. How do I indicate that particular post - other than saying "check out post #78 in www.- - - - post. ???

You report it from the particular posting (your example, go to post 78, thread XYZ, then press the triangle).
It sends a note to the moderators where it was reported from.