Your ADE (Adobe Digital Editions) log in and password needs to be the exact same ones as the Sony Library to read the DRM protected files. Also, your computer and Sony 300 need to be registered/authorized through ADE and the Sony Library.
You can check MR's (MobileRead) Wiki (at the top of this page on the left side) for additional information about
ADE and
ascm files (
DRM protected files).