After the condemnation, Italy and Japan withdrew from the League of Nations.
Other nations began to test its power and Germany also withrdrew
Germany withdrew from the league in 1933 on the grounds that the League did not treat them fairly.
Japan
japan
Germany withdrew from the League of Nations on the 19th of September, 1933.
There was no such thing as the United Nations. You are confusing the League of Nations with the UN. Germany was in the League for a short time in the 1920's but withdrew in 1933 when Hitler became Chancellor.
Original Answer: Because they are decades away. Improved: The United Nations are not a country. They couldn't have "joined" the League of Nations. The League of Nations was the predecessor of the United Nations - the League of Nations failed, and so, many years later in 1942 (I believe it was that year) they founded the United Nations as an improved "League of Nations."
Germany
The acquisitions (mandates) no longer really governed themselves. They were administered by nations on behalf of the League of Nations.
Yes, the Headquarters of the League of Nations was in Geneva, Switzerland.
United States of America