You did say 'intel' macbook pro right, otherwise if not an intel mac it won't run on OSX 10.5
You need Java 1.6 (Java 6) to use LW 5. The way to check is go to the Utilities folder, open Java Preferences. Now, if you do not see this version of Java you cannot use LW 5. However you will probably see multiple versions of Java listed in the order of preference of use. Put Java 1.6 (Java 6) at the top of the list. Play around with the order & see if this helps.

I'd hate to think you'd have to use LW 4 instead. But this might be an option.