I looked for a source for this quote some time ago for a different thread. Here's what I found:
"After getting feedback for a story idea from his artists, he isolated
himself to map it out over 13 weeks of dailies and Sundays (1953
article), with the playwriting formula 'First act, get your leading
character up a tree; second act, throw rocks at him;
third act, get him down.'"
I don't know if it originates with Allen Saunders, but it seems to be a well-known writing formula already at the time. I have seen it attributed to various writers, not just Nabokov.
Joseph Schlegel
PhD Candidate
Department of Slavic Languages and Literatures
University of Toronto