What you are describing is a slide lock, mainly to be used while traveling to stabilize the top of the slide and seal the slide better. You can also use it during storage to get a tighter seal. There is no lock for the bottom, the slide mechanism takes care of that even though the label you are referring to says "top and bottom" which is a bit confusing. As for the placement of the slidelock, it goes in the opening at the top of the center cabinet, I put mine at an angle, the end that goes against the trailer wall being the high side of the angle and the side against the slide being lower. The ends of the slide lock will move back and forth when turned one way and will stay in place the other way. When placing the slide lock , make sure the ends are in the lock position, up and down or the lock will tend to push out when you tighten it and not hold.
And BTW, welcome top the forum!
2004 Chevy 2500HD Crew Cab 6.0l, 4.10r
2010 Jayco Jayflight 28BHS
Reese Strait Line with Dual Cam
P3 Brk controller