That's not it. The "snap helper" grid relates to the placement of windows. The "grid desktop" activity has to do with the placement of plasma widgets.
I no longer have Slackware installed, so I can't show you the activity, but I can show you where I'd find it if I had it on my Kubuntu Desktop.
http://dl.dropbox.com/u/10167641/images/snapshot.png
CORRECTION: In the ever-shifting, ever-confusing lexicon of KDE4, it used to be an "activity". Now it's a "layout", and I'd find it in in the "layout" list along with "Desktop", "Folder View", "Newspaper Layout", and "Search and Launch"