Music Creator 6 start up dialog box

I bought MC6 yesterday.  When I start it up I get a dialog box pop up "Cannot open MasterIns.  No instrument definitions will be available.
 
??
 
Thanks

Welcome to the forum.
That shouldn't be a problem unless you own one of the MIDI keyboards on the list and were planning on using that keyboard's sounds.
Every Cakewalk program I own has a small number of instrument definitions included. Instrument definitions are lists of how the sounds on a particular keyboard are mapped. So if you have a Roland Juno and it's on the list and selected under Options>Instruments, you can use the patch names of your keyboard in MC or SONAR and the program will play that patch on the keyboard.
Music Creator cannot import new IDs, it's always been like that, but programs further up the line can import and use custom IDs.
So don't worry about it and if there is a check box to 'not show this message again' go ahead and check it so you won't get that message every time you open MC.