CNET currently does not offer previous versions for download, though I've heard they would like to offer that feature in the future. For now, though, you can only read about the previous version, not download it, which is why you only found the "Download the latest version" link, along with in-page advertisements. When the download is available, the Download Now link is prominently displayed until the software's title, as is the case for Posterino 2.0.1, the latest version. You'll need to search other sites for Posterino 1.6.3, being careful regarding which ones you trust, but you may also want to check CNET Downloads every now and then to see if the 'download older version' option has been added yet.

Regards,
John