You need to add a reference to the library you're using into Visual Studio, so it knows about that code.
Something like this:
In Solution Explorer, double-click the My Project node for the project. In the Project Designer, click the References tab. Click the Add button to open the Add Reference dialog box. In the Add Reference dialog box, select the tab indicating the type of component you want to reference (.Net) Click Browse Navigate to the DLL provided by Stimulsoft.