When you purchase Pro, you will receive an email from sales/customer support with a link to your personal pro download page. I always recommend people Bookmark that page.

To check if LW 5 is compatible on your computer, check to see if you have Java 6 (1.6) installed because LW 5 requires java 6; Setting up Java for Leopard (see 2nd image)

If you do not have Java 6 listed then you are restricted to LW 4. To install LW 4, you will first need to remove the LW 5 install package file from Receipts folder else the system will not let you install an earlier version; Receipts folder sample image