I’ve found, if you have the camera flash output set to Manual (1/128th power), the ACC button on the strobe needs to be ‘IN’.
If by contrast you have the camera flash output to eTTL, then the ACC button should be ‘OUT’. You are basically telling the strobe that there will be a preflash and to ignore this and fire on the main flash.
Old thread alert, I know. Sorry, but wanted to chime in.
I was trying to get my Inon's to work with fiber optics because I have way to many failures with the wired cables. Alex's explanation above was the only way I could get the strobes to be controllable on manual. Set the D7100 to manual 128th, push in the ACC, adjust away. Opposite with TTL, set the pop up flash to TTL, release the ACC to the out position, set to a tty mode, done. Near perfect TTL.
PS, not sure why I am showing up as a guest. I am logged in. Guess I need to come around more often.