To be a little more specific, you can find the Layers dialog in two places under the "Windows" tab at the top. One is under Recently Closed Dialogs, and the other is under Dockable Dialogs. Windows > Recently Closed Dialogs > Layers, Undo, etc. results in a dialog box displaying not only the...
Bear with me here... because I don't use mac. Neither does lozzy, however.
Open up GIMP and click on File > Preferences. There should be an option that says "Folders". If I'm right, there should be a folder in there that has the path name(where your brushes folder is located). You can add...
From what you've said it sounds like you actually tried to hit each separate key. You don't have to press "+" or anything like that. Just hold down CTRL and then hit C. This is the keyboard shortcut for Right click > copy. Holding down CTRL and pressing A is the keyboard shortcut for Right click...