I believe this is because your project is referencing the .NET Framework 4.0. Open your Project properties and click on the References tab. Highlight all of the ESRI references. In the Properties window set Embed Interop Types to False. You really should change your project to reference the .NET Framework 3.5 since 4.0 is not supported by ESRI at ArcGIS 10 and you may run into other problems.