Output Hub will not allow you to download an expansion pack if it detects that the main engine is not currently installed. If you've recently updated or done a re-install of Output Hub it may not save what you have installed. This also applies if you've installed the engine using Native Access.
To resolve the issue click the download button, and when prompted, point it to the directory where your library is installed.
If you used Native Access to install then engine then check the following directory:
MAC: Macintosh HD > Users > Shared
Hub will check the file structure of the Library and will shortly display as installed if everything is updated and correct.
For the FX plug-ins, redownload the engine and then cancel out of the installer to make display as installed.