I am trying to set a filter rule that will match only messages that
are "ignored"

Here are the steps I followed:
Create a new filter or edit an existing filter
Select "Article score is at least"
Select "Low" from the drop down list
Add New Line to the Filter
A new line appears with description "Article score one of: watched,
high, medium, zero, low"
Select the new line and click on "Invert"
Line description changes to "Article score on of: ignored"
Click on Apply
Click on OK
Go back into the filter and the line has the  description "Article score one of: watched,
high, medium, zero, low"

This was a little disconcerting at first, as I use a filter of ignored
to delete incoming articles - couldn't work out why I wasn't seeing
any new messages! ;-)

Note: There is a workaround for this, as you can manually edit the
filters.xml file.

I am using version 0.14.0 on Mandrake 9.0. It is based pm the packaged
rpm from the pan site. I use the -nospell option.

Regards,
Dave