The Notes data directory is located by default at

C:\Program Files\IBM\Notes\Data or
C:\Program Files (x86)\IBM\Notes\Data.

However, if you placed it elsewhere, or Notes was installed in multiuser mode on the workstation, you can easily find it from the User Preferences dialog box.

Open User Preferences.

The data path is shown in the field Local application folder. Do not alter this field or Notes will fail. Right click in the field, choose Select all, right click again and choose Copy. The data path will now be on the clipboard to paste where you need.