If you need to be attacking with an item in order to get benefits from its properties, those properties will always explicitly say so. Some examples of items like that include the Flaming Weapon and the Thunder Wand.
Flaming Weapon +1
Power (At-Will * Fire): Free Action. All untyped damage dealt by this weapon changes to fire damage. Another free action returns the damage to normal.
Italics emphasis added.
Thunder Wand +1
Property: Gain a +1 item bonus to damage rolls when you use this wand to attack with a power that has the implement and thunder keywords.
Italics emphasis added
The property of the Staff of Sleep and Charm has no such requirement, and thus it applies even when wielded in the off-hand, without requiring you to attack with it. It's very popular in character optimization for certain classes and builds (e.g. Bards, Enchanter Wizards) because of this.
Staff of Sleep and Charm +1
Property: You gain a +1 item bonus to attack rolls with arcane charm or sleep attack powers.
Note that the properties of any weapon or implement will still only function while the item is wielded (i.e. held in one or both hands like a weapon), even if they don't require you to attack with them, unless stated otherwise. Enhancement bonuses and critical bonus damage depend on the weapon being used to attack, as well as any applicable weapon or superior implement properties.