To Add a Toolbox

In the Customize dialog ( File > Settings > Configuration > Customize ) , click the Tools tab.
Expand the User Tools section.
Do one of the following:

Select the open DGN file.

or
Expand the open DGN file and select the toolbox to which you are adding the new toolbox.
Do one of the following:

Click the New Toolbox icon.

or
Right-click the open DGN file or toolbox and select New Toolbox from the pop-up menu.

A new toolbox appears with the name "New Toolbox."
Type a name for the new toolbox.
(Optional) Select the new toolbox. In the Properties pane, click the arrow to the right of the Dimension field to change it from "Both" to "2D" or "3D."