Is your machine an intel Mac? If not, you will be unable to use LW 5 & be restricted to LW 4, unless you upgrade to Snow Leopard 10.6.

(Command-Shift-4 for a snapshot of your choice of area using the cursor; ie: once you press those 3 keys you will see a cross-hair cursor, keep holding down the Command-Shift keys and drag cursor around your preferred area to snapshot, then let go of mouse & snapshot will be taken. Snapshots are usually saved onto Desktop.)

(I've moved your thread to the Mac OSX installation problems section)