As you see I load the libtdsodbc.dylib upfront, so I can detect if library loading fails upfront.
Than I tell SQL Plugin to use that library and use the ODBC connecting string as you see above.
i just checked, and although this may well be complicating it for no reason. 
the port (1433) is for the default instance only.
any named instances will usually be listening on a dynamic port number.
Thank you Christian & Russ.
The Port caused the issue. I am now also using 1422 and it works. 
Is it true that if more then 1 client in the same Network on different Machines use my app now and all with this Port Setting, this makes no difference for the Server? Multiple Clients can connect to 1 Server by using 1433 as Port on the Clients?
As far as i remember, only multiple instances on 1 machine could get in trouble if all instances use the same Port. Yes?
when you connect to 1433 the system will actually choose a different port.
The advertised port is just the one you send the connect package. the reply will than switch you to the actual port which is different for each connection.
Thank you Christian. 
If you’re still having trouble, you might try ActualTechnologies drivers for OS X. They have an ODBC driver that works wonderfully with SQL Server. I used these at my previous job…
Thank you Greg, it works wonderful with MBS Plugins. But the more alternatives for our community, the better
One additional information which may hopefully be helpful for someone in the future.
If you want to place the libtdsodbc.dylib into the Apps Resources Folder (Mac) within the Bundle, add a Build Step to copy the File (Build & Debug Mode) to the Resources Folder and use the following code to access it:
[code]dim libtdsodbc as FolderItem
libtdsodbc = App.ExecutableFile.Parent.Parent.Child(“Resources”).Child(“libtdsodbc.dylib”)
Dim s As New SoftDeclareMBS
Call s.LoadLibrary(libtdsodbc.NativePath)[/code]
Maybe you want to add this to your Blog Article Christian?
I’m not even sure you need the SoftDeclare thing. On Linux we needed it to load the referenced lib first, but on Mac we have only one.