To compile projects using Grids in MonoDevelop, you have to set your assembly options target framework to Mono/ .NET 4 for all the assemblies in your project (typically, Assembly-CSharp and Assembly-CSharp-Editor). You can find the settings by right-clicking on the assembly in MonoDevelop, then clicking on Options. Then select Build|General, and change the target framework to Mono/.NET 4.

setting the target platform