Oh sorry.. did you try this method? from the help file... (below). I don't know the syntax for C#, but there are some C#-related code samples in this forum and articles that may have some pointers, this one comes to mind:
http://www.mp2kmag.com/a103--chandu.....mappoint.html

SetFieldsVisibleInBalloon method
Sets which fields are visible in Pushpin balloons for the specified data set.

Applies to
Objects: DataSet

Syntax
object.SetFieldsVisibleInBalloon(ArrayOfFields)

Parameters
Part Description
object Required. An expression that returns a DataSet object.
ArrayOfFields Required Variant. An array that contains the Field objects to display in the Pushpin balloons.

Example
Sub ShowOnlySomeFields()
Dim objApp As New MapPoint.Application
Dim objDataSet As MapPoint.DataSet
objApp.Visible = True
objApp.UserControl = True

Set objDataSet = objApp.OpenMap(objApp.Path & "\Samples\Clients.ptm").DataSets(1)
arArray = Array(objDataSet.Fields(1), objDataSet.Fields(2), objDataSet.Fields(3))

'Set which fields will be visible in the Pushpin balloon
objDataSet.SetFieldsVisibleInBalloon arArray

'Show the balloon for the first record
objDataSet.QueryAllRecords.Pushpin.BalloonState = geoDisplayBalloon
End Sub

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