Cannot edit metadata in Compressor.

I would suggest rather than exporting a file from FCP and then recompressing it, that you directly use the Send to Compressor function in the File menu. That will send the project to Compressor and let you set whatever settings you want to use. I've never tried what you suggested, but I don't see why it would make a difference to editing the metadata. I can do it like any other text box, click and delete or type additional text.

Could you post a screenshot of what you’re talking about and where? What metadata do you want to enter, where?

That's strange. Did that metadata come with the clip or did you add it? I can edit even camera metadata. Try rebooting the system.

Did you use Send to Compressor? I cannot reproduce this. All the the metadata sent from FCP is editable in the Job pane.