Once you add them, toolboxes appear in the left pane on the Tools tab in the Customize dialog. The toolboxes in the open DGN file appear below the blue V8 icon. You can name your toolboxes and organize them by moving them up or down. A toolbox can contain other toolboxes as well as tools. The toolboxes in the open DGN file can be opened, moved, copied, deleted, and renamed.

Once you have added a custom toolbox it appears in the Tool Boxes dialog and you can open it from that dialog.