As simple as the summary says:  find usages does not work -- it does not find usages.  I can consistently reproduce this
as follows:

1.  Create a new NetBeans project for existing sources (I have tried "freeform" and also "with existing sources" with
the same result).
2.  Choose a public member of a class which is known to be used by another class within the same project.  Right-click
and choose "find usages".
3.  Accept the find usages dialog (I have tried setting scope to current project or all projects; and checking or
unchecking "search in comments" with the same results).
4.  Result -- the progress bar appears only for a small fraction of a second (whereas when find usages works as
expected, it's obvious because the progress bar takes a while), and then the usages window shows "0 occurrences.

Note that if the member appears in the currently opened class, then the usages in the current class will be found, but
usages in other classes will not be found.

Also note that if I open another class which uses the member, and then I return to the original member and repeat the
"find usages", this time it will find the usage which appears in the newly opened class.  So it appears that NetBeans is
only becoming aware of the usages of a certain class after I explicitly open that class.