I've read and re-read the National Weather Service's formula to convert millibars to inches of mercury, however, trying to get something to work in excel is beyond my scope of simple formulas. I have searched many sites for help, but it seems nothing is out there. Anyone know of any hidden formulas someplace?

Google says, that
1 inhg = 33.863886666667 millibar,
or 1 millibar = 0.02953 inhg

and I tried...I must be doing an incorrect formula, or incorrectly.

In cell A1 have your millibar value.
In any other cell, the formula:
=A1*0.02953


If you wanted to be silly and use Excel's built-in conversion function:
=CONVERT(CONVERT(A1,"hPa","mmHg"),"mm","in")