Error 2 The type or namespace name 'HelloForm' could not be found (are you missing a using directive or an assembly reference?)
I'm willing to bet you're missing a using directive. Once you add the assembly reference, you also need a using statement at the top so your program knows what namespace the control lives in.
Alternatively, you could use the fully qualified name, which is the namespace followed by the object, to declare it.
You'll have to find out what the namespace is, then put a using directive at the top of your code file.