I’m trying to adjust the clutch on my dt360a.
I’ve loosened the clutch cable adjuster at the hand lever.
when I turn the adjusting screw at the engine, supposedly to get the actuating arm 10* off centre, all that happens is the clutch rod moves down out of the engine. The actuating arm doesn’t move backwards or forwards the required 10*
any help is appreciated.

We might need a bit more explanation.
But There's a Titty on the adjuster big screw that engages in the actuator arm.
Once engaged there's probably only about one full turn when engaged before said titty bottoms out in the arm groove.
So if it bottoms out then go back anti clock to max adjustment which i think on yours will be down on the arm not up as on later top mount ones.
Lowering it will move the arm back tightening the cable.
But if its not happening do you have the clutch apart or the pressure plate off so the rod has nothing to push on which in turn makes the arm move back on the adjuster screw.
Just remember you only get half a turn from min to max adjustment by lift & lowering.
Keep the screw as deeply engages as you can as they pull the thread when tightening the locknut which only needs nipping up.
It runs a soft-ish washer behind the nut & a seal in the nut outer end to seal on the screw head part.
If i'm off the planet someone will haul me in. I'll go lookning at cmsl or partzilla now just to be sure.
One more thought--have you had the clutch pressure plate off & not engaged it in the correct orientation.
Has 3 arrows & 3 marks on the hub face to make it easy to fit on so it goes right on & compresses the plates.

Thank you.
the clutch etc is fully installed on a running bike.
turning the adjusting screw in or out only raised or lowers the arm, the lever doesn’t move forwards or backwards the 10* specified.

OK so I guess the clutch pushrod is in there plus any ball bearing spacer there's meant to be. If you hold your finger on the arm & move it to touch the pushrod while turning that adjuster left & right the half turn to raise or lower the arm you should feel it move back--tight cable. ,or forward-- slack cable. Not a great deal of adjustment down there. If that doesn't work then back to the drawing board. Cmsl doesn't show the usual 5/16 ball between the mushroom and pushrod so I'm guessing your rod must have rounded ends, if the parts drawing is correct.

As the lever drops down out of the case you are reducing internal freeplay. 

I prefer to disconnect the cable completely at the bottom lever.  Makes it a little easier to feel internal freeplay when pressing lever with finger as RT325 described. 

So I disconnect the cable at the arm and hold it towards the rear and turn the adjusting screw in the necessary direction to have the arm/lever 10* to the rear?

No, if the cable pulls from the front, which I think it does, you'll be holding the arm forward so it contacts the rod. If there's a curly spring on the arm it'll be trying to pull the arm back but those springs are pretty useless so may not be doing anything. Can I ask how all this started. Is the clutch not operating. What do you feel when pulling the lever at the handle bars. That lower adjustment isn't magic & is quite minimal if you have a stretched cable or something unusual going on. Think those cables had a midway adjuster if getting rid of freeplay is the main issue.

Hope i'm not misunderstanding what youre saying.
Not sure what you mean by 10* but lever is #20 i'm talking about.
www.partzilla.com/catalog/yamaha/motorcycle/1974/dt360a/clutch
Re-thinking things if your aim is to get the arm as far back as possible to get rid of lots of free cable then all you need to do is turn the adjuster to set the arm at the lowest point & that's the maximum you can go.
You can see by #20 that at the lowest point its closer to the rod on this model so moves the arm back where cable conects.
If you find you can't connect the cable-ie too short-- then you've gone too far for the cable so need to go back a bit. But before going back just see if there's cable adjustment to back off first to help it reach.

There is plenty of free play adjustment at the hand lever. The clutch does drag a bit if you try to push the bike in gear with the clutch lever pulled right in, or if you try to kick start it in gear with the hand lever pulled. I would like to have all the adjustments correct.
I just don’t understand how to get the 10* adjustment.
In the manual, it just says turn the adjusting screw in or out to get the 10*. It doesn’t even mention that the clutch actuator rod moves down as you adjust the screw.

Yes part number 20 is correct.
the right angle at the bottom is what I think the 10* refers to.
from a centre line the right angle bit at the bottom is 10* behind centre and 10* in front of centre when the clutch hand lever is pulled