The iOS version 3.06 release notes list the ability to save custom data from a PC to the cloud and loading it from the cloud to an iPad. I haven´t been able to find where this function can be activated. Can anyone help out?

You didn't mention in which product you couldn't find the option. In SkyDemon Plan it's under the Setup menu, in the section about Custom Data. In iOS/Android it's under the Setup menu, in the Cloud dropdown.

Thanks tim, found it. Curious to understand what sort of custom data one would use this feature for. Is it intended for aircraft attributes such as speed, fuel consumption, weight & balance and so forth?

No, it's for people who create actual airfields, airspace, VRPs, that sort of thing. Their own charts, basically. Some users have a need to express other airspace "entities" beyond standard aeronautical data on the charts due to their specific mission profiles.