I have purchased RealFlight 8 Horizon Hobby Edition or the RealFlight 8 Horizon Hobby Edition Add-On disc, however, I don't see all of the aircraft listed on the box in the aircraft selections drop-down.

The remaining aircraft are available as part of the Day One Patch for the RealFlight 8 Horizon Hobby Edition. To update your software, click on the version number that is found directly beneath the ‘Run RealFlight’ menu button. If no version number is present, you already have the most current software available (assuming that your machine is connected to the internet).