here is the info from Section 2 -
Quick Tip - Black Keys & Non-Standard Note
A black note to the right of a white key is shown using a '+' symbol eg 1+ is the black key to the right of note 1 (called C# in traditional notation).
A black note to the left of a white key is shown using a '-' symbol eg 7- is the black key to the left of note 7 (called B♭ in traditional notation).
So 2+ is the black note to the right of note 2.
More information on playing Fur Elise is in section 5d - Classical (ClassicPlay Method).