When referencing another sheet with Data Validation,
you'll need to assign a Range Name to the source list.

Example:
To use a list on Sheet2 for a DV on Sheet1

Select the source list range on Sheet2.

From the Excel Main Menu: <insert><names><define>
Names in Workbook: (assign a descriptive name....like: myDVList)
Refers to: (make sure the proper range is selected)
Click [OK]

Then on Sheet1
<data><validation>
Allow: List
Source: (press [F3] to see a list of Range Names...select the one you need)
...finish the DV settings and Click [OK].

Does that help?
Post back if you have more questions.
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Regards,

Ron
Microsoft MVP (Excel)
(XL2003, Win XP)

Hopefully this helps:

Make a named range of this particular range in the Sheet2
In the Data Validation window, as cursor is blinking in the field source
press F3 for the named ranges
From the list choose the Range Name for the source to be used
Choose OK

Regards,

Mika Oukka
IT-Consultant
Finland, Helsinki