Then you're probably looking for one of these:
- online interpreter for MscGen - mscgen_js
- atom package for writing & previewing MscGen - mscgen-preview
- command line interface - mscgenjs-cli or
- how to embed MscGen in html
(At this spot might arise the interactive interpreter currently residing at mscgen.js.org and sverweij.github.io/mscgen_js one day).