In Word 2003, I am trying to convert a letter into a document that I can use
for a mail merge. On the toolbar, the "insert Word field" button is active.
However, the button to its left, the "insert merge fields" button is
greyed-out or inactive.

I want to insert address merge fields into this doucment. How can I make
the "insert merge fields" button light up so I can use it?