The Cybook Opus supports Mobipocket format books natively, with no need to upgrade any of the software. You will however need to ensure that you have registered the Mobipocket PID for the device prior to downloading your purchases.
Here's where you can find your PID:
You can then log in to your eBooks.com account and download your Mobipocket format book to read on your Cybook Opus.
The Cybook Opus can also be used to read unprotected PDF and EPUB files. These files can simply be transferred from your PC to your handheld device.
For further help with the Cybook Opus, please click here.