It says that you are using a deprecated type that might not be supported in a future release of Visual Studio.
DAO seems to have been deprecated by Microsoft for quite a while. A quick Google search shows people having your problem as early as .NET 2003. Obviously they don't want people to use it anymore and rather encourage them to switch to ADO.NET or something else. So if you are not locked into DAO, I'd recommend exploring other database options.
However, as CDaoDatabase
is still present in the documentation for VS 2012 and even 2013 Preview I think you can safely ignore that warning if you really have to use DAO. Seems like it will be supported for a little while longer.