The DataRowCollection.Item property requires an integer for the row index.
I think you are after the following syntax:
txtAddress.Text = DB.ProfileDataset.Tables("tblCustomers").Rows(0)("Address").ToString()
EDIT
Something to keep in mind:
original code
= DB.ProfileDataset.Tables("tblCustomers").Rows.Item("Address").toString
compiler sees
= DB.ProfileDataset.Tables.Item("tblCustomers").Rows.Item("Address").toString
fixed code
= DB.ProfileDataset.Tables("tblCustomers").Rows(0)("Address").ToString()
compiler sees
= DB.ProfileDataset.Tables.Item("tblCustomers").Rows.Item(0).Item("Address").ToString()