Unfortunately Unity has stopped supporting Monodevelop-Unity on 2018.1.0 release. However you can still use Monodevelop-Unity by downloading the Unity Installer of version 2017.4.7 from this link: https://unity3d.com/get-unity/download/archive , choosing custom install and selecting to install only Monodevelop. After it’s installed in your PC, you can select it as the script editor in any version of Unity.

https://answers.unity.com/questions/1520926/how-to-download-monodevelop-and-use-it-in-unity.html