I have the shop manual for a similar engine which also specifies this
tool.  In the manual there is a picture of it.
It appears to be a sleeve with outside threads at the business end,
which you would twist into the seal and thus pull it axially.
The process of using a tool like this probably completely destroys the
old seal, but you're pulling it anyway.  Just be sure to have its
successor on hand.

The old shade-tree method, of just prying the old one out using a very
smooth-sided tool (so as not to nick any sealing metal surfaces),
would probably work just fine too, and is in fact what I'm about to do
this weekend.  Having done this before I am comfortable with the
technique, but warn others that if they nick the sealing surfaces the
new seal will be damaged and/or not work.  Proceed at your own risk.