imac 20? What's that? Which model imac do you have? Is it an intel imac or not? Do you have java 1.6 (6) on your system? If not you will need to use LW 4 instead. How to check? See Setting up Java for Leopard (see 2nd image) & if your java preferences does not list Java 6 (1.6) then you cannot use LW 5, but instead, LW 4.