Hello jtalac, and welcome,
The DataPump is running on the server, under a service account which might have different privileges to what you see on the desktop. It's possible that the external lookup data source is visible to you on the desktop, but not to the service account. One odd thing worth checking is that you aren't using a drive mapping to a network share like X:\Accounts\Files\Lookup.xls as the service account won't have drives mapped to it. You'll need to use the UNC path like
fileserver.company.com\data\accounts\files\lookup.xls - you can manage these references in Monarch using the Monarch Utility.