Did you first create the Data Validation dropdowns as instructed?
Since you're using another workbook for the source, make sure you use a
named range from that workbook as your list source.
http://www.contextures.on.ca/xlDataVal05.html
A sample workbook can be found at Debra's site
http://www.contextures.on.ca/excelfiles.html#DataVal
DV0028 - Data Validation Combobox Named Lists -- Double-click a cell that
contains a data validation list, and a combobox appears -- font size can be
set, more than 8 rows displayed, autocomplete can be enabled. Lists are
stored in named ranges on a separate sheet. Macros must be enabled.
DataValComboboxSheet.zip 13 kb updated 15-Jan-07 View detailed
Instructions
Another one from same site.............
DV0043 - Data Validation Combobox With Entry Check -- Double-click a cell
that contains a data validation list, and a combobox appears -- font size
can be set, more than 8 rows displayed, autocomplete can be enabled. When
the combobox loses focus, the entry is validated. DataValComboCheck.zip 15
kb 29-Jun-06 Updated 11-Oct-06
Gord