As I recall, a little play is normal. That is, mine has some play.

The rod is attached to a diaphragm. If you turn it you will tear
the diaphragm and break it. To avoid that, you must loosen the lock
nuts and turn one rod on the end without turning the portion that
goes into the diaphragm. Then tighten back the lock nuts. You
must be careful that the end going into the diaphragm does not spin
(use a wrench on it's nut which is attached to the rod).

To correctly set it up you hook up a pressure gauge and air pump
and you pump it up to a known air pressure (the value varies
by model and engine) and then check to see if the diaphragm is
engaging. I believe IPD published the procedure in one of their
recent newsletters (www.ipdusa.com). You could ask them to fax
you a copy (they are well known for customer service and just might
do this).

Volvo also has a Technical Service Bulletin that has the exact
procedure. You can subscribe to www.alldatadiy.com to get the
TSBs (my subscription just expired so I don;t have the TSB number).
Volvo's procedure references Volvo tools (but you can substitute
your own air gauge with plumbing fittings from the local hardware
store).

There are also quite a few posts here on this topic and you can
do a search to see how others have done this. I think many just
turn it a bit and experiment with it. Just limit it to about 3
turns.