In the end I do not think it would have accomplished anything.
Blowing up the Hoover Damn would hurt a small portion of America. The US's resources were spread out all over the United States.
A "terrorist" attack would have put fear into the people, but in the end it would have only rallied the US people even more and given them more resolve to end the war and more than likely "punish" Germany on an even greater scale.
I would agree with D.A.I.G. on this.
Instead Adolf should have send Brezels and Beer to the USA since 1933, reminding the US about its historical ties (e.g. Steuben) and population heritage to Germany, the common issues between Nazis and KKK or the general affiliation of the southern states to Nazi Germany.
Who knows, but there might have been a chance to disrupt the US – England affiliation, rather then ignorantly declaring war against the largest economic power in the world.
Regards
Kruska