I think i have a solution for you.
I can't really see the install process on your video, but if it shows, that it will install about <10MB or less (or at least significantly less than 34MB, which is the POD HD 500 Edit.app size), then you'll have to remove the remained .bom file in the /private/var/db/receipts directory. Usually these files contain a list with all the files that get installed within the install process, so the installer can completely remove or update them.
I've had happened to had installed serveral versions of all POD HD Edits and i also went into this problem. The uninstall process seems to not properly work.
So you can check if you have a /private/var/db/receipts/com.line6.application.podhd500edit.package.bom on your disk but not an HD Edit 500.app, then you'll have to remove it first, otherwise you cannot install a new one.
The best way is, use the Terminal (Applications->Utilities->Terminal) and remove the file with sudo. It think easier is to tell the finder the proper directory, so use Shift-Command-G then enter:
This will open a Finder Window in the proper directory. Now look for two files named
and remove them. If that doesn't work you might try the terminal and use sudo. Or ask me again.