Hi ch4meleon,
Slightly easier than Moira's suggestion:
Click on the mail merge field and press Shift-F9 to reveal the code. The field will look something like:
{MERGEFIELD "YourDate"}
Edit it to look more like:
{MERGEFIELD "YourDate" \@ "MMMM d, yyyy"}
then press Shift-F9 to toggle the code display off before running your mail merge.
With the date format switch (\@ "MMMM d, yyyy"):
. 'yyyy' outputs the year and century (4 digits);
. 'yy' outputs the year only (2 digits, including leading 0s);
. 'MMMM' outputs the month in full;
. 'MMM' outputs the month as a 3-letter abbreviation;
. 'MM' outputs the month as 2 digits (ie with leading 0s);
. 'M' outputs the month as 1 or 2 digits (ie without leading 0s);
. don't use 'm' instead of 'M' for months - 'm' is for minutes!;
. 'dd' outputs the day as 2 digits (ie with leading 0s); and
. 'd' outputs the day as 1 or 2 digits (ie without leading 0s).
Cheers
PS: You probably don't need the \* MERGEFORMAT switch.