Some quotes and a proceedureal summary from the Sept 1999 Cadalyst article by Mark Middlebrook.
Quote:- "The DesignCenter window is nonmodal —it doesn't interrupt other AutoCAD commands, but waits to one side for you to request something."
Quote:- "Rather than use Load, you can drag a drawing file from Windows Explorer into the DesignCenter window. Be careful, you must display the Desktop view in the tree view pane and you must drop the DWG filename in the palette or tree view pane. If these conditions aren't met, AutoCAD opens the drawing in the drawing editor rather than in DesignCenter."
Quote:- "One option to consider is just to:-
1. Open a drawing containing named objects that you want to copy and then open (or create) a second drawing into which you want to copy the named objects.
2. Then click the DesignCenter's Open Drawings button to display your two currently opened drawings in DesignCenter's (tree)navigation pane on the left.
3. Click the Layers table to display the source drawing's layers in the content pane.
4. Choose one or more layers in the content pane.
5. Right-click in the content pane and choose Copy from the cursor menu to copy the layer(s) to the Windows clipboard.
6. Click in the AutoCAD destination drawing's window (the drawing that you opened in step 3).
7. Right-click and choose Paste from the cursor menu.
AutoCAD copies the layers into the current drawing using the colors, linetypes, and other settings from the source drawing.