I have a custom arrangment set up for viewing newsgroups. Go to View >
Arrange By > Edit Custom Arrangements, and then click "New". My arrangement
looks like this:
Custom Arrangement Name: Public Newsgroups
Group items by: Subject
Sort groups by: Sent Newest on top
Sort items within groups by: Sent Oldest on top
Default display for groups: Expanded
Click OK. Now, open a newsgroup, and then go to View > Arrange By > Public
Newsgroups. That might provide threading that you like. Otherwise, you can
go back to the "Edit Custom Arrangements" dialog and make changes as you
wish.
There is no way to do true threading (by reference), which shows
specifically which messages in a thread are replies to which. This is a
commonly requested feature for Entourage which hopefully will be implemented
in a future release.