This is a tricky one and something which took me ages to work out when trying this before.
In your add-in add a function like this in a standard module:-
Public Function ReturnClass() As ProjectName.Class1
Set ReturnClass = New Class1
End Function
Where ProjectName is the name of your add-in as specified in the Properties window when you select the add-in.
Now, in the workbook where you want to use the classes try this:-
Select a reference to the add-in from Tools, References. Then you'll be able to do this:-
Dim c As ProjectName.Class1
Sub test()
Set c = ProjectName.ReturnClass
c.DoSomething
End Sub
I don't believe you can directly create classes defined in another project, but this seems to work well enough. Please let me know if something is unclear.
Regards,
D