Right click References, Add Reference
select the COM tab in the dialog box and locate your activeX object - I'll call mine objA for now - and hit Ok. objA should appear in your list of references.
In your code you will need to put
Then you can use it like any other object e.g.
objA a = new objA();
When you build your project you will get an additional file called Interop.objA.dll which will need to live in the same folder as your exe
I've successfully used this approach with several legacy VB6 COM DLLs and EXEs - but apologies for any typos!