On ours your can activate it just using the power switch, but it's not obvious. You switch on, wait for the display to say "Powersave off", switch off, and then switch on again. Now it'll say "powersave on". Beware that there's a small time window somewhere in the startup sequence during which, if you switch off and on again it gets mighty confused and then cycles between powersave off and powersave on forever without starting up properly. If you get to this state you can recover just by cutting the incoming 12v with the isolator for a few seconds.
The jumper/dipswitch changes the default state at power up, so you don't have to go through this malarkey every time you switch on. I can't remember the details and I can't get into mine to look without undoing all the connections and mounting nuts. As far as I can remember, I never found anything online about it, so it must have been fairly obvious from the markings on the PCB which jumper for switch does this.
If I were you, I'd try the on-off-on technique first, to test that the powersave mode works ok with your gear, and if it does, isolate everything, remove the cover and look for a set of jumpers or dipswitches one of which is marked in some unambiguous way.
HTH
MP.
ETA. Page 10 in here https://www.donrowe.com/v/vspfiles/pdf/prosine_1000_1800_owners_guide.pdf