You can download previous versions of PLECS Blockset at the following link. The link is also accessible at the bottom of the main PLECS Blockset download page.
https://www.plexim.com/download/blockset/archive
I would recommend downloading the latest version of that release, PLECS 4.5.9, as it will include additional bug-fixes. If you need 4.5.6 for a specific reason, then please use this link.