Step 1: put both tables in the same dataset, if they are not already.

Step 2: define a Relation in that dataset, linking the two tables by idMaster.

Step 3: add a column to the Details table, with an expression displaying a field
from the parent table.

Step 4: add a column to your grid to display that field.

So if you start with

Dim Master As DataTable
Dim Details As DataTable

you can then do

Dim MyDataSet As New DataSet

MyDataSet.Tables.Add Master
MyDataSet.Tables.Add Detail

MyDataSet.Relations.Add( _
   New DataRelation("MasterToDetail", _
   Master.Columns("idMaster"), _
   Details.Columns("idMaster")))

Details.Columns.Add( _
   New DataColumn("MasterName", _
   GetType(String), _
   "Parent(MasterToDetail).masterName"))

Voila. You can now display the MasterName column of the Details table.