The SharePoint 2010 (Client API) Provider has an inbuilt feature to automatically match and link Lookup Columns based on the value associated with the List.
In this example we have created an Orders list and added one column that is a multi-value column that will contain the 'Products' that relate to this Order.
To handle the lookup data we simply need to provide the name of the lookup value separated with a semi-colon(;) i.e. here we created an Excel file with the order details.
We then simply create a DS Project to Map this to our Orders List
Then once we synchronise this data the lookup is automatically connected.
During the Synchronisation we download the list items from the related SharePoint List and index into this list to get SharePoint's internal identifier number to assign to the item being linked. Therefore items must be an exact match and they must exist for the link to be successfully created. If your Lookup is simply a single item then you just need to return the Name/Title of that item.