Unfortunately file extensions in Windows are hidden by default, but you can see them in the command prompt (or you can change the settings of Windows Explorer).

Since skindle.exe is in the same folder as book1.awz, the only way I can see you can get the "skindle is not recognized as an internal or external command, operable program or batch file" error is because you run skindle from the directory where there is no skindle.exe.

So open the command prompt and go to downloads\skindle-06
(cd downloads\skindle-06) and then run