Mail Merge Fields Grey and not selectable

I don't understand why anyone would want to use the standard method in Word 2013.  It is a lot of manual data entry. 

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I created mail merge documents years ago, I think with Word 2007, which operates the way I want. Word 2013 doesn't do it this way, which I dislike.  The new method doesn't make it easy..

You are mistaken. All Word versions work this way, going back at least 20 years. There is nothing new about Word 2013 in that respect.

A different approach that you might find useful is to copy one of your prospective Excel source files and give it a generic name (e.g. Mailmerge.xlsx) in the folder where you keep the mailmerge main document (or another common folder). Then set up the mailmerge in Word to point to that file. Insert all the mergefields etc, as you desire. From then on, any time you want to create a new mailmerge, copy the chosen clinet's file to Mailmerge.xlsx, then open the Word document and run the mailmerge. This way, there's no need to select a new/different data source each time from within Word.

Not sure if this solves your problem, but have you tried the Insert tab's Quick Parts > Field option. If you do this and select MergeField as the field type, you can then specify your own merge field name to be inserted into the text. I use this to create document templates that I can then populate from an external program.