I presume you do not actually get ALL zeros.
What number did you type into the cell, is it possible that you have
Lotus 1-2-3 Transitions on in Tools, options, transition and are
doing a subtraction. I guess you said you only see zeros.
Anyway the zip code formats in Format, cells, special
require you to choose 5 digit or 9 digit, seems you chose 9 digits
and entered 5. and your actual format would be 00000-0000
so a zip code of 07710 would look like 00000-7710
I would suggest that you for the column as Text and then you can include
zip codes from any county, and sort your US zip codes properly.
http://www.mvps.org/dmcritchie/excel/join.htm#fixUSzip5