This problem can be solved through three steps:
First Step:
Edit > Preferences > External Tools > External Script Editor
Then just choose the option as it is in the image:
WhenyougettothisstepjustchoosetheMonoDevelopoption.
SecondStage:
IfinthefirststepyoudidnotseetheMonoDevelophypothesistochoose,youhavetoclickwhereitsaysBrowse...,andgotothefolderwhereMonoDevelopisinstalledandchooseitsexecutable.
ThirdStage:
IfyoudonotfindthefolderortheMonoDevelopexecutableinStepTwo,youmayhavechosennottoinstallitduringtheinstallationofUnity3d,inwhichcaseyouwillneedtodownloadMonoDevelopandinstallit,thendotheSecondandFirstStepdescribedhere.
MonoDevelopDownload: Click Here!
This download, although it says XamarinStudio, is the normal MonoDevelop, it just has to do with companies and their businesses, but its functionality is the normal MonoDevelop, the IDE of C #, F # and so on. If after installing it is named XamarinStudio it will have the same functions as MonoDevelop, so you can do the Stages as I explained that the only thing that will change are the MonoDevelop names for XamarinStudio.
PS: The option that has a certain as it is in the image is the one that is chosen and selected, in my case I preferred Visual Studio 2015, but to have MonoDevelop, .
I hope I have helped.