To Select a Model as a Cell

Select Drawing > Annotate > Cells dialog launcher.
The Cell Library dialog opens.
From the Cell Library dialog's File menu, choose Attach.
The Attach Cell Library dialog opens.
Select the DGN file containing the required model (to be placed as a cell) and click OK.
The dialog closes and the DGN file is loaded as a cell library. Models that can be placed as cells are displayed in the list box.
Select the required model and click Set Active Cell icon to make it the active cell.
Use the Place Active Cell tool to place the model as a cell.