You will need to create some empty formulas in the report header sections and then you should be able to use code pass in the field values from the dataset tables. These formulas become part of a collection and you can reference the collection by name (a string) or the index number (an integer). For simplicities sake I always use the name.
Without looking, I believe this is done using the datadefinitions properties, though the .Net help file will give you the correct syntax for the object model.
Perhaps others can help with the explanation or correct any errors I may have made.
Apart from that, I hope this gets you pointed to the solution you need.
Regards,
John W.