First, start Stardock Central > use the "Tools \ Register Products" > enter your Object Desktop serial number (if not already entered for you) > click the "Update" button. This will allow you to download the full version of WB5 if, as Snidley indicated, you are current.

Note - make sure that the "Work Offline" under the File menu is not selected, otherwise you won't see anything but installed full versions and shareware versions offered for the paid products.

Note #2 - unloading and/or uninstalling WB4 before installing WB5 may be required.