Has the ADL supported resolutions acknowledging acts of genocide and resolutions condemning genocide denial before?

Yes. A partial list of U.S. Congressional and international resolutions that the ADL has supported includes:

July 22, 2004
Concurrent House Resolution 467 and Senate Resolution 133:
“Congress declares that the atrocities unfolding in Darfur, Sudan, are genocide, and urges the Administration to refer to such atrocities as genocide.”

July 23, 2004
ADL Press statement regarding Resolutions 467/133:
The ADL commended Congress for condemning the atrocities being committed in the Darfur region of Sudan as "genocide."

"We applaud Congress for this bipartisan effort to marshal support for a more robust U.S. response to the genocidal atrocities in Darfur," said Barbara B. Balser, ADL National Chair, and Abraham H. Foxman, ADL National Director. "As Jews, we are acutely aware of the lesson of the Holocaust, to never again remain silent in the face of such acts. We are confronting a crisis which challenges us to action while there is still time, and not to delay as in the case of Rwanda. Time is running out, but American pressure can still make a difference and reduce the death toll in Darfur."

January, 26 2007
United Nations General Assembly Resolution denouncing denial of the Holocaust (UNGA Resolution 60/7)

January 23, 2007
In a letter addressed to countries that had not endorsed the resolution (China, Egypt, Jordan, India, Saudi Arabia, South Africa, United Arab Emirates, Qatar, Brazil and Japan), the ADL wrote:

"We urge you to support this important declaration by the international community reinforcing that it will never forget the Holocaust and rejecting those who seek to deny it. Furthermore, such a declaration is critical to ensuring that the world does not ignore current and future acts of genocide."

Given the universal truth of the last statement – that by denying any genocide, we risk future genocides – the ADL’s stand on the Armenian Genocide is unconscionable.