Walking on cellulose or fiberglass
In short, you don't want to walk on ANY type of insulation. If you compress it, it will lose its R-value (i.e., its insulation value.)
If I were you, I'd use fiberglass batt insulation. Yes, it can be moved easily so you can walk up there. If you're considering later on of using the 'space' up there for storage, I'd build a 'dummy floor'. You simply build floor joists in a box-shape, and lay the insulation INSIDE the joist bays. See what I mean??? Then, you can screw down some plywood on top of the box, and store stuff on top. (You only need a few screws in case you need to 'lift' the dummy sub-floor on top of the joists. Don't use nails ...)
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!