You're doing this the hard way.

Try it this way:

Kmenu>System>More Programs>File Manager Super User Mode

This should prompt you for the root password and then present you with a Konqueror Window.

Now, click your way to the directory in which file you want to change is, right click on the file, select properties and the permissions tab. At the bottom you can change ownership, while the advanced permissions button enables you to select who can execute it.