I'm in learning mode - so my aplogies if this is a super-noob question. I'm watching the vids I found here http://wikihelp.autodesk.com/Infrastructure_Modele
Thanks and have a great week.
It popped up for me no problem.
Perhaps you are zoomed out to far? Just a guess.
After it's created, if you want to change it, grab the outline of the area and not one of the individual trees and you should see it then too.
You need to select the whole "forest" for the slider to appear: this can be achieved by first selecting a tree of the forest by using left-mouse-click and then doing another left-mouse-click on it again (note: don't try it with a double-left-mouse-click!)...
Here's what I'm seeing http://www.screencast.com/t/tLunlpCen
Full-disclosure: I JUST installed this as a trial version and have not registered yet. I don't have the Autodesk BIM 360 installed or anything. Usually that wouldn't be an issue - but you never know what detail will make a difference.