Connect options greyed out

Am trying to connect a page element to a dataset. Having clicked on the “Connect to Data” option, and selected a dataset, I am then looking for the “Connect Options” - the one I want is a field called “Document”, which has a document in it from my Media folder. However, that field is greyed out. The field called “Title” is available, as are the hidden fields. Could anyone please tell me why the “Document” field is greyed out?

Thank you
Graham

Simplicity itself - as a newbie, I had failed to read the documentation properly, where it clearly says to put an image placeholder on the page first. Must do better!

I don’t understand. Please try explaining better.