Slavoj Žižek criticizes Buddhism in two ways: by ridiculing the Dalai Lama's dictum that Buddhism makes you happy—My God, hasn't this man heard about Freud?—and by referencing the support afforded to nationalist militarism in WW II by the Japanese zen sects, citing in particular laudatory remarks expressed by D.T. Suzuki.
The second argument is more easily dispensed with: the political ideologies associated formerly with the zen establishment in Japan have as much relation to the teachings of the Buddha as the Spanish Inquisition does with the teachings of Christ. The abuse and perversion of religious ideals by institutional authority is a sad and ancient story, but there is nothing especially Buddhist about it. Indeed in all the Asian countries where Buddhism flourished, there are remarkably few instances.
The Dalai Lama however can be criticized for advertising the view that Buddhism makes you happy, giving critics the chance to speak of it as "fast-food religion," or see it as simplistic New Age-ism. It's not that Buddhism doesn't make you happy—why would so many people practice it for 2.5 millennia if it didn't?—the question is rather whether the Buddha actually taught that and that the answer is clearly no.
The original teachings of the Buddha—as closely as we can discover them through philological analysis of the earliest layers of the Pali Canon sutras— are contained in the Four Noble Truths and Eightfold Path. These teachings say that suffering exists, that the unreflected pursuit of desires is responsible, and by committing to certain ethical precepts and to the practice of meditation, such suffering can be overcome. This is anything but some "Don't worry, be happy" species of spiritual dumbdown. In truth the practice of Buddhism is really quite challenging.
Slavoj Žižek would be better advised to view the Buddha as a kindred spirit, since he was in fact an early philosopher working at the dawn of human literacy, addressing themes common to the Greek Pre-Socratics, and, as evidenced by the early Pali Canon narratives which show him in dialectical exchange with all kinds of contemporary philosophers, ready and eager to respond to any kind of criticism.