From KnifeEdge, the explanation of the free Add-Ons:
RealFlight Add-Ons 1 & 2 have been repackaged for RealFlight G4. Even better, they are now available as free downloads! Numerous technical issues prevent the original Add-Ons disks from working with G4, but because we want G4 users to have access to this content, we're giving it away. The aircraft have been re-trimmed and the airports have been tweaked to look more like the original G1/G2 versions. Also, in cases where we updated an AO1 or AO2 aircraft (e.g. the Piper Cub) and made it a RealFlight G4 aircraft, the old version does not appear in the repackaged Add-Ons. All of the Add-Ons content is otherwise unmodified.
The repackaged Add-Ons volumes are only compatible with G4 version 4.00.051 or later. They will not work with RealFlight G3/G3.5, which already fully support the original Add-Ons disks (still available for purchase). This content is released as-is and we do not plan to make any further changes to it.
Add-Ons 3, 4, and 5 will also be re-packaged for G4 and released for free when they are ready.
http://www.knifeedge.com/forums/ look under News and Announcements