When you have a RadSpreadsheet control on an ASPX page or an ASCX usercontrol, you may observe the following error:
"Could not load file or assembly 'Telerik.Web.Spreadsheet, Version=XXXX.X.XXX.XX, Culture=neutral, PublicKeyToken=121fae78165ba3d4' or one of its dependencies. The system cannot find the file specified."
The above error indicates that the Telerik.Web.Spreadsheet.dll, which is required by RadSpreadsheet, is not referenced by the project. In order for RadSpreadsheet to work properly, you need to include and reference this assembly.
You can find the Telerik.Web.Spreadsheet.dll in the AdditionalLibraries folder in the Telerik UI for ASP.NET AJAX installation (automated and manual) and in the internal builds archive. It supports .NET 4 and .NET 4.5, so depending on the target framework of your project, you need to take the assembly from the Bin40 or Bin45 folder respectively.