I had a similar problem. I haven't tried this yet, but I'm hoping it's the solution:
- Start the Sony reader software.
- Plug in your reader
- Once the computer recognises the usb connection, start ADE
- IN THEORY, ADE then recognises the PRS-300.
I haven't managed to try this out, because my husband's been hogging the home computer and my work computer is running XP 64-bit