Your question is a little unclear as to what you mean by 'automatically find and enter these counties'.
Do you mean create a list somewhere on the worksheet that contains all county names included in a particular zip code?
If so, you can use a macro to do that. The following macro assumes the Zip Codes are in column A and Counties in column B.
If you wish to use different columns for your specific situation, modify the macro:
Change both "A" references to the column letter your zip codes are in, i.e. "C"
Change the "B" reference to the column letter your counties are in, i.e. "D"
Change the "L", "L1", and "L:L" references to the column you wish results to appear, i.e. "M", "M1", and "M:M" respectively.
Then copy this macro, modified if desired, to the clipboard:
Sub CountybyZip()
Dim i, LastRow
LastRow = Range("A" & Rows.Count).End(xlUp).Row
Columns("L:L").ClearContents
selZip = InputBox("Please enter a zip code to extract.", "Zip Code")
Range("L1").Value = "Counties for Zip Code " & selZip
For i = 1 To LastRow
If Cells(i, "A").Value = Val(selZip) Then
Range("L" & Rows.Count).End(xlUp).Offset(1, 0).Value = _
Cells(i, "B").Value
End If
Next
Columns("L:L").AutoFit
Columns("L:L").HorizontalAlignment = xlCenter
End Sub
Press ALT + F11
In the menus at the top of the VBE, select INSERT > MODULE
Paste the macro into the editing area to the right.
Close the VBE and return to the worksheet.
Press ALT + F8
When the Macros window opens, highlight the macro and click 'Options..
Enter a letter to be used as a keyboard shortcut and click 'OK'.
Close the Macros window.
Press CTRL + your shortcut letter to run the macro.
Enter a zip code and press Enter.