Unit uHYIntf not found (nor uHYIntf_FMX)

I am new to Hydra, and try to create a FireMonkey visual plugin for a VCL host, using custom interfaces. I am using the trial version.

I get this error message:

[dcc32 Fatal Error] uCustomInterfaces.pas(7): F2613 Unit 'uHYIntf' not found.

I also try to change this into uHYIntf_FMX, but also with no luck. Searching on my disk for these two files also reveals nothing. Is this a limitation in the trial version, or is a general limitation?

this error can be raised in trial version if your plugin was built without Hydra_VCL or Hydra_FMX package.

as a workaround, your plugin should use above package