David,
The actual basis for this upload is the associated Web Service - in this case, ManageLeadsIn.
You can look this up within the Service Explorer - you'll then need to download the WSDL and open it in a program that can read it (SOAPUI is a good choice).
Within this, you'll see what fields are available.
I did this, but i don't see the "Marketing Unit" within the WSDL. I see the following:
MarketingResponsibleEmployeeParty - this may be the closest that you can send as it relates to a marketing party, but appears to only accept an Employee as the value. You should see a corresponding field in the Excel XML schema.
SalesResponsibleEmployeeParty
ProspectParty