Go to “Your Username/Library/Preferences” and locate the folder which is named the same as your Cubase/Nuendo version.
Please note: Since Mac OS X 10.7 (Lion), the user’s Library folder is hidden! Here is how to access it: In the menu bar of Mac OS X click on “Go”. While the menu is folded out holding down the [ alt ] key (options) will show the “Library” entry.