If you just need it to display with the dash, choose the appropriate
format from the Format/Cells/Number/Special dialog.

If you actually need the - inserted, one way:

B1: =TEXT(A1,"00000-0000")

In article ,
Angela wrote:

I have zip codes in a field fields that I need to add a dash between the
first part of the zip and the second four numbers so the instead of looking
like "902101234", it looks like "90210-1234". How do I do this?