Anselm's theory can justify the existence of anything. The perfect island may involve glowing sand, a glistening sea on the horizon, dancers, 50's music, and cocktails. Taking Anselm's approach, this island therefore exists, it's the most perfect island that I can conceive of. Howver the island does not exist anywhere else but in my mind
However, according to Anselm, his theory only works for necessary beings, of which God is
However, God being a necessary being depends on God existing in the first place, and therefore can't be used to justify the existence of God