This problem can be solved through three steps:
First Stage:
Edit > Preferences > External Tools > External Script Editor
Then just choose the option as it is in the image:
When you get to this step just choose the option Monodevelop.
Second Stage:
If the Monodevelop hypothesis for choice did not appear in the first step, you will need to click where it says "Browse...", and go to the folder where you have installed Monodevelop and choose its executable.
Third Stage:
If in Step Two you do not find either the folder or the Monodevelop executable, it may be because you chose not to install it during the Unity3d installation, in which case you will need to Download Monodevelop and install itYou will then have to do the Second and First Stages described here.
Download Monodevelop: I clicked Here!
This Download despite saying Xamarinstudio, is the normal Monodevelop, has only to do with companies and their businesses, but its functionality is that of the normal Monodevelop, the IDE of C#, F# and etc. If after installing it gets the name of Xamarinstudio it will have the same functions of Monodevelop, then you can do the Steps as I explained that the only thing that will change are the names of Monodevelop 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, just click on it.
I hope I’ve helped.