About Einstein 

My search of any document that might bear Einstein's name - in the League of Nations archive - in between 1930 - 1932 returns no results. See http://biblio-archive.unog.ch/archivplan.aspx.  By reviewing five books dealing with Einstein's work and life - not a single one mentioned this Einstein's support to the Ustase cause. Just one, written in German,

Einstein: Sein Leben by Denis Brian, Christine von Samson-Mark, Wiley-VCH, 2005 page 352

Indem er sich den Angelegenheiten seines eigenen Landes zuwandte, empoerte sich Einstein ueber deutsche Studenten, die fuer die Entlassung eines Professors agitierten, weil dieser ein Pazifist gegen politisch motivierte Todesurteile war. "Es ist entsertzlich zu sehen, wie die unerfahrene Jugend fehlgeleitet wird", schrieb er an das Berliner Tageblatt. "Wenn dies so weitergeht, werden wir 
nach einer Tyrannei der Faschisten bei einer Herrschaft des Rotten Terrors ankommen." Er glaubte, dass eine solche Herrschaft bereits in Jugoslawien ausgeloest worden sei und kritisierte die Landesregierung fuer den Mord an dem kroatischen Fuehrer, Professor Milan Sufflay, der Geschichte an der Universitaet von Zagreb gelehrt hatte.

Reading further this book it is clear that the only information available to Einstein was 'The "inside story" of the Šufflay murder was given by the Croat journalist (Ustase propagandist), Slavko Cihlar, in the Berliner Tageblatt, February 26, 1931.'

So, it is apparent that Einstein fell under spell of the Ustase propaganda. About the same propaganda, see:

Appeal of the Croatian Academicians to the World of Civilisation: How the Croatian Savant, Professor of University, Dr. Milan Sufflay was Murdered by the Serbian Royal Dictatorship
by Croatian University clubs association, Zagreb, 1931 32 pages

which was elaborated by 

"Blood and Homeland": Eugenics and Racial Nationalism in Central and Southeast Europe, 1900-1940
by Marius Turda, Paul Weindling, Central European University Press, 2007 page 107

Shortly afterwards, students from the Croatian University Club Association produced an English language pamphlet, How the Croatian Savant, Professor of University Dr. Milan Šufflay, was Murdered by the Serbian Royal Dictatorship, in which they appealed to European public opinion to recognize that the murder of Šufflay was the result of endeavors to build an absolutist South Slav state. In such a state, the "European culture" of Croatia and the Croatian was to be replaced by Serbian suzerainty, in which "orthodox, byzantine, oriental-asiatic and oldtuc (sic) political and social traditions" had been subsumed into a system "not only contrary but also odious to European culture"

Turda also says that "His murder occasioned international condemnation and outrage from, among others, Albert Einstein and Heinrich Mann of the German League for the Rights of Man" (no source was given)

About the Sufflay murderer

Thinking about Yugoslavia: Scholarly Debates about the Yugoslav Breakup and the Wars in Bosnia and Kosovo by Sabrina P. Ramet, Cambridge University Press, 2005 page 254

Goldstein (Croatian historian) leaves the question of agency to the side and writes simply that Šufflay, who, as he notes, was an accomplished observer of Albanian culture, 'was killed on the street in Zagreb. The killers were never found" see also: Croatia: A History
by Ivo Goldstein, translated by Nikolina Jovanovic,  McGill-Queen's Press - MQUP, 1999 - page 125

About the same Sufflay

Women, Gender, and Fascism in Europe, 1919-45: 1919-1945 By Kevin Passmore, Manchester University Press, 2003 page 115, Fascism in Croatia chapter: Ante Starcevic and Milan Šufflay, who used a demonic concept of Serbs as the cornerstone of their ideologies.

Conflicts: Studies in Contemporary History by Lewis Bernstein Namier, Macmillan, 1942 page 49

The racial theories of a Croat writer named Šufflay were taken up since Sufflay had held that the Croats were not Slavs like the Serbs, bu were descendants of a settlement of Goths.

War and Revolution in Yugoslavia, 1941-1945: Occupation and Collaboration by Jozo Tomasevich
Published by Stanford University Press, 2001 page 20:
Sufflay was indicted in 1921 for espionage and connections with Croatian emigres and sentenced to athree year prison term

Bottom line: the story about Einstein's support to the Ustase cause is one-sided and even false.72.75.20.29