It's because MonoDevelop may have created the MyClass class in another namespace which has the same name as your solution.
So either:
- right-click "MyClass" in your code and search for something like "resolve" (but I've found right-clicking can freeze your IDE :()
add a using statement at the top of your file, e.g. if your solution is named "GtkTests":
using GtkTests
instead of MyClass use <SolutionName>.MyClass:
GtkTests.MyClass MyClassInstance1 = new GtkTests.MyClass();