Something is not right on your system. A program in ~/.local/bin/ should be on your path. If not, then the Rapid Photo Downloader installer will have created a link to it in ~/bin. In all cases, ~/.local/bin/ and/or ~/bin must be in your path–if they’re not, you’ve got a problem.
Did you do a clean install of LinuxMint, or an upgrade?
Those instructions apply only to those installing the application on other distributions. I’ll look at clarifying that. It seems the different heading levels don’t make it clear enough 