It doesn't.

These applications have only been uploaded to Chinook Extras (and in ScummVM's case, Diablo Extras-devel). Unfortunately their developers have not yet seen fit to get them into DIablo Extras. So when you download that .install file from Downloads, the Application Manager goes looking for the package in Diablo Extras (since you're running Diablo) and can't find anything. Thus, "application package not found".

There are two possible workarounds for this. First option, find and harass the maintainers of those packages to get their packages into Diablo Extras. Second option, add Chinook Extras to your application catalog.