how does Zaroff lure men into the island?

The General lures the ships toward the island with lights, misleading the crew into believing there is a safe harbor...... when there isn't.

From the text:

The general's left eyelid fluttered down in a wink. "This island is called Ship Trap," he answered. "Sometimes an angry god of the high seas sends them to me. Sometimes, when Providence is not so kind, I help Providence a bit. Come to the window with me."

Rainsford went to the window and looked out toward the sea.

"Watch! Out there!" exclaimed the general, pointing into the night. Rainsford's eyes saw only blackness, and then, as the general pressed a button, far out to sea Rainsford saw the flash of lights.

The general chuckled. "They indicate a channel," he said, "where there's none; giant rocks with razor edges crouch like a sea monster with wide-open jaws. They can crush a ship as easily as I crush this nut." He dropped a walnut on the hardwood floor and brought his heel grinding down on it. "Oh, yes," he said, casually, as if in answer to a question, "I have electricity. We try to be civilized here."