Thank again . Refer my reply below in Bold
Since ListBasedWebPart is the base class for the cascading menu web part, not seeing it in the intellisense off "using" statement is correct in your VS.NET file. Am I right by assuming the screen shot illustrates your code behind file for your custom master page?[Yes, you are right, code behind for my master page] It seems to me that you are able to declaratively include qCascadingMenu in master page using SPD 2010 but failed if you are programmatically including it in the master page code behind in your VS.NET solution[Yes, you are right]. Please clarify if this is the case.
If that is the case, with "using WA.Common" is not good enough. You will need "using WA.Navigation" and add WA.Navigation.dll to project reference. This assembly contains the "CascadingMenu" web part.
I have inluded WA.Navigation in the solution. Still I am getting the same error.
Even I tried other WA's dlls also, (in case..) but no sucess.
May be the WA.Navigation is not updated in version 5.7, but I am not sure.
To cut the story short, still stuck up. 🙂