The Munich agreement was appeasement in order to avoid a world war (which ended up happening anyways due to Hitler not following through on his promises).

Meanwhile the USSR and Nazi Germany cooperated to invade and divide Poland with the Molotov-Ribbentrop Pact.
They also cooperated in military matters; allowing Germany to train and test equipment in Russia to avoid the Versailles Treaties provisions.

It wasn't like the Allies actively tried to work with Hitler they just wanted to avoid a war and maintain the peace. While the USSR wanted to work/use Germany to reclaim territory lost from WWI and gain new ones as well.