To Add a Custom Library (AutoCAD Mechanical Toolset)

Click Content tabLibrary panelContent Manager. Find
In the Content Manager dialog box, in the Content panel, click .
In the Content Manager - File Locations dialog box, under Custom Content, click in the last empty cell in the Library file column.
Click to browse to the folder containing the content library and select the .GDB file in that folder.
When a valid .GDB is selected, the name of the selected library from the .GDB file populates the Custom Content grid.
Click the Save Changes and Close button.