Run Program from Browser/File Manager

Ok, I am pretty new to Linux, and I am having a problem finding programs from the File Explorer. I am currently using Ubuntu.

The real problem I am having is that when I download a file (such as a torrent) and I want to open it with a different program than the default (I want KTorrent instead of BitTorrent). When I pick choose program it takes me to a file explorer. I type in the name of the program (KTorrent) and it says:

"The application you chose ("(null)") could not be found. Check the file name or choose another application."

I get the same message when I copy the command from the link (torrent %i %m -caption "%c" %u) and try to execute that.

So, where is the directory of executables? Where do I need to point Firefox? And how do I change the default for opening different file types? I tried "Preferred Applications" but this only lets me change my browser, email client, and terminal emulator.

First firefox is not really suited to be a file manager although it will browse
directories. If your going to execute files and so on I would recommend
Konqueror for KDE and Nautilus for Gnome.
Most all of the executables are in /usr/bin

Unfortunately, this feature is not very well designed. You must
already know where the application's executable is in
order to tell the file manager that this is the app to open a particular
type of file. I hope it improves in the future.

Objuan: I'm not trying to execute a file from Firefox, just set a file to open with a specific program when I download it in firefox.

Does anyone know where KTorrent keeps its executable and what it is called?

The exectuable for most programs is its name on /usr/bin

/usr/bin/ktorrent

But some KDE programs use
/usr/kde/<your version>/bin