I think the earlier intel machines were not 64-bit capable. Just to check, go to Apple menu top left of menu bar, choose Software update ... and see if any Java updates. Then if installations or not, go to Utilities folder & open up Java preferences; Setting up Java for Leopard (the 2nd image). If you see no Java 1.6 (6) listed in your Java preferences, then your computer is not 64-bit java compatible & that's why you cannot install LW. I hope that's not the case, but let's see.

If Java 6 (1.6) is listed, then move it up the list & try again with installing.

Frankly, there's several functions missing from LW 5 that were in LW 4. Many people have complained as it's not easy to navigate as before. It looks quite different.