The IconSet and Material class are part of the IconSet quark, and should be installed if you’ve installed the NodeSnapshot quark - because they’re marked as a dependency. If you’re having “missing class” errors related to IconSet and Material, make sure the quark is actually installed (Quarks.installed.do(_.postln);).
I pushed a fix for the isFolder issue - you can update via Quark("IconSet").update;.
If you still have issues after updating, let me know exactly what error messages you’re seeing and what led to them? Apart from the isFolder issue, I can’t reproduce any problems on a clean local SC installation.