After installing ADE, open the program and connect your eReader to the computer via USB.
Locate your downloaded Epub file in ADE and move it to your connected device.
(Note: A small number of eReaders may not work with ADE, so you will have to use that eReader's software to transfer your eBook file)
To transfer from a computer to your eReader device via direct copy:
Request a download code email, and follow the instructions to save the Epub file to your computer.
Connect your device to your computer using a USB cable. Your eReader now appear either in My Computer (for Windows) or on your Desktop (for OSX).
Copy your downloaded Epub file to your device by putting it in the correct directory:
For the Nook, locate your Nook's "My Documents" folder.
For the Kobo, locate your Kobo's "Digital Editions" folder.
For Sony Readers, locate your Sony Reader's "Books" folder.