Preload Adjuster Use on SR Suntour XCM
Turning the preloaded dial towards the negative symbol will decrease the preload on the spring. Therefore if you find the forks are too stiff, backing off the preload will soften the fork. Alternatively if you wish to firm up the fork, turn the preload dial towards the positive symbol. This will compress the spring and stiffen up the suspension. 

I would adjust the preload with the lock out off, though I don't believe it would make any difference beside allowing you to compress the fork as you dial preload on/off to see what affect it has on the fork. 

Hope that helps!