Before you use the toolbox to drag the sub form control onto the main form,
in the upper right corner next to the arrow click to toggle wizards on/off.
Dragging the name of the form from the Database window onto the other form
(which is open in design view) should work.
To open another from from a command button, just use the OpenForm action. If
you want them to stay synchronized, see: http://support.microsoft.com/?id=210060