Whey and Yeast
According to the website Bukisa, ladybugs seem to be attracted to the scent created by a mixture of whey and yeast. You can combine these two ingredients and spray them onto plants to which you hope to attract ladybugs. Using a mixture of sugar and water with the yeast, instead of whey, also attracts ladybugs.
Other Ladybugs
Ladybugs living in a comfortable environment will secrete a pheromone. Other ladybugs smell the pheromone and flock to the area.
Aphids
Because ladybugs prey on aphids, they are naturally attracted to the scent they give off. Ladybugs will converge on any area where they smell aphids, knowing there is a food supply. There are also several types of ladybug attractant sprays available that mimic the smell aphids produce.