It was an excellent idea for Apple to include an Arabic - PC keyboard layout in Snow Leopard. But one has to wonder where they got the mapping they used. A poster in the Apple forums pointed out that the character Thal ذ (U+0630) is at an obscure non-standard location (Option/alt + 3) and the character Shadda ّ (U+0651) is not present at all, at least on an ANSI keyboard.
Normally, as shown here, these characters are on the leftmost key of the top row. On an ISO (102-key) layout, they should be next to Return.
For an alternative layout that I think comes closer to what PC users expect, try this one.
Oddly enough, the Arabic - PC layout that was included in Leopard is OK.