That's because a selected data key can contain multiple different things, so the SelectedDataHelper only has functions to get lists of things.
You can simply access whatever is stored in a key like this:
object data = dataHandler.Get("key");
dataHandler is a SelectedDataHandler instance.
Since data could be anything, you'd have to check what it is, or directly do it like this:
EquipShortcut equip = dataHandler.Get("key") as EquipShortcut
equip would be null if it isn't an instance of EquipShortcut.
Otherwise, use the helper class, e.g.:
List<EquipShortcut> equipment = SelectedDataHelper.GetEquipment(dataHandler.Get("key"));