For all of those people who are having issues with Mechjeb, MAKE SURE that you have updated the toolbar addon as well. KSP Interstellar bundles what looks like a seriously outdated version of the Toolbar, and it looks like Mechjeb does not support that version. It took me about two hours of troubleshooting to figure it out, but once I updated the toolbar mod, it worked perfectly. Sarbian, might want to put that in your main post, and poke FractalUK about updating the version he ships Interstellar with.
