http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/Mede...
"We conclude that neither Avena nor the President’s Memorandum constitutes directly enforceable federal law that pre-empts state limitations on the filing of successive habeas petitions."
If they want this to become law in the US, they will have to follow the usual process. If they want criminal sanctions without trials, as the treaty proposes, the Constitution will have to be rewritten. (The fact that enforcing these laws will be technically impossible is another annoying issue.)