I just started learning unity and I created a project. But my project doesn't have a sln file included in. Every time I create a C# script and open it in Xamarin Studio I can't have any intellisense.
If "Open C# Project" doesnt create the .sln file, try updating your external tools first.
In Unity, go to Edit > Preferences, and make sure that Visual Studio is selected as your preferred external editor.
This created the sln file for me.