You may open an archive using drag and drop by selecting it in the Windows Explorer holding down the left or right mouse button, and move the mouse cursor over the BitZipper main window and release the button.
If no other archive is open in BitZipper, either button works similar. If another archive is open and you use the left mouse button, the dragged archive will be added to the already open archive. If you use the right mouse button, a popup menu is displayed when you release the mouse button, allowing you to select whether to add the file to the active archive or to open it in another window.