Hi,
Assuming that your DataSet object exposes a table in a straightfoward manner and that you have your BizTalk schema defined already, in your VS project, after you create a Map, pull the web port (that references your web service response message) as your Source schema, and the BizTalk schema as your Target schema.
Then, on your map grid, drag and drop a Looping Functoid onto it, link the the left-hand side of the functoid to the parent node of your web port schema (that represents a record node) and link the right-hand side of the functoid to the record node of your BizTalk schema. Afterwards, you can link the fields from the web port schema directly to the elements of the BizTalk schema.
Open the properties area of the Map, and make sure you test its functionality by setting its input and output, to verify that things will be fine, before you compile and deploy it.
I hope this is enough information to get you started.
Daniel.