how to use mute in string instruments?

If you mean, how do you add the text to your score, this would be staff text - Ctrl+T, or from the text palette. The General MIDI standard doesn't include a definition for muted strings, though, so you won't get any special playback effect unless you find, install, and use a custom soundfont that includes appropriate samples. See the Handbook section on soundfonts for more info.