LW 5 will work on Snow Leopard. However you should make sure you have all your Java up to date; OSX Software Updater.

However when you suggest you have an old power pc mac, then I'm thinking you don't have OSX 10.6. Knowing which OSX version is vital to know which version of LW will operate on it. If using OSX 10.4 or earlier you are restricted to LW 4 versions. Use the first link Birdy gave to check this.

If still problems, give us some system details & tell us what you have tried so far.