When you drop a command button on the document it puts you in Design Mode. Click the first button on the Control Toolbox to toggle this mode. When not in design mode you can protect the form.

You'll also need to do something like this to toggle the protection off while adding the row:

\nPrivate Sub CommandButton1_Click()\nActiveDocument.Unprotect\nActiveDocument.Tables(1).Rows.Add\n' ... add form fields here ...\nActiveDocument.Protect Type:=wdAllowOnlyFormFields, NoReset:=True\nEnd Sub\n

Edit: Which could be a problem if you are password protecting the document. If you just want to keep the casual misplaced cursor from doing damage to the form, then it should be OK.

This was all in Word 2002 (aka XP)