。
Mother forgot to add salt to the salad.
Literal
Mother [topic-は] salad [to-に] salt [obj-を] add [nominalizer-の] [obj-を] forgot.
のを忘れる nominalizes the verb clause (加える → 加えるの, 'the act of adding') so it can become the grammatical object of 忘れる. English uses an infinitive 'to add' for this; Japanese packages the whole clause as a noun phrase marked with を. This is a core pattern for any verb that takes an action as its object.