Let's define objective correlative as objects, situations, and/or events which function as a formula for a particular emotion. This formula, presented as external facts, evokes that particular emotion. The situations in "This is Just to Say" are the speaker eating (and enjoying) the plums and his subsequent (alleged) guilt for having done so. The question is: what situations and objects evoke these emotions? Although the subjects of the poem are the speaker and the reader (of the note), the plums are the object focused on.
Often, a visual description establishes an objective correlative which evokes an emotion. In this poem, Williams uses the form of an ordinary note to write an evocative poem. The first two stanzas seem very commonplace; much like a note ("I did this/you did that.") The last stanza is different because in asking for forgiveness, he combines emotion with sensual pleasure. The speaker asks for forgiveness but then justifies his act. A more ordinary ending might have been, simply, "Sorry." But the speaker ends with,
they were delicious
and so cold.
Note the sound of the word delicious (and "plums" for that matter). Both words involve facial movements (in pronunciation) which evoke the act of eating (and therefore, enjoying). These words are onomatopoetic in the sense that they sound like what they describe. Also note the repetition of the consonants (alliteration) and vowels (assonance) in this last stanza: the "s" in "delicious" and "so sweet" as well as the "o" in "so cold." These adjectives put the focus on the taste of the plums.
The plums are the objects and the objective correlative is the description of enjoying them. Although the point of the poem is to ask forgiveness, the formula (objective correlative) evokes the sense of taste as much as (if not more than) the emotion of guilt/asking forgiveness. In fact, many words in the poem relate to eating: plums, eaten, icebox, breakfast, delicious, sweet, cold.
The feelings evoked by the objective correlative can be emotional as well as sensual (taste in this case). One conclusion might be that even an ordinary note can contain evocative and/or meaningful sentiments.