If the DLL is a COM component, then make sure it is registered first using Regsvr32.exe, and then add a reference to it in your project through the COM tab.
Alternatively, you can use P/Invoke. E.g.:
[DllImport("csp2.dll", EntryPoint="FUNCTION_NAME_IN_DLL")]
public static extern void MyImportedFunction();
Read more on
Platform Invoke.