I doubt this may help but... ours is a HappiJac circa 2001.
Handle must be released or will not move..
However I read on your linked pdf about second person holding power switch after releasing? I don't know what that is about-sounds like a brake- ours you mechanically disengage- it free wheels-no second person doing anything or accessing switches so yours may differ. Just on the off case thinking handle possibly either broke, missing or just hidden....
This is taken laying on floor, slide is in.
When slide is out cant see this, access is thru floor. The sub floor cut out, just visible in top right of picture, aligns over motor when slide is out, removing a carpeted board gives access. Ours is full wall. Sounds like yours is partial slide-access thru seat cabinet.
This just shows lever flipped to disengage position. As the lever is on bottom of unit-when looking thru from floor side-its barley visible.