Adding rows in datagridviews

You've said you want to calculate the addition on two rows so....I'm not sure exactly what you are having trouble with, but to access the data in a row, you would access the row's cell's value something like this (Assuming that the data you want is in the first row and first column):
dgvGrid.Rows(0).Cells(0).Value

So if you had two rows in the grid and only one column you would add them together like this (Assuming that the cells contain decimal values):

Dim decAddedValue As Decimal = dgvGrid.Rows(0).Cells(0).Value + dgvGrid.Rows(1).Cells(0).Value

Your best bet would be to create a function that makes sure a value IS what you expect it to be and/or returns a converted value...for example, it may be possible to have a null value in your cell, in which case the above code will fail. I'd do something like this:

Private Sub btnCalc_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalc.Click
    Dim decAddedValue As Decimal = CvtObjDec(dgvGrid.Rows(0).Cells(0).Value) + CvtObjDec(dgvGrid.Rows(1).Cells(0).Value)
    MsgBox("Value is: " & decAddedValue.ToString)
End Sub

Public Function CvtObjDec(ByVal obj As Object) As Decimal
   If obj Is DBNull.Value Then Return 0
   If obj Is Nothing Then Return 0
   If obj.ToString.Trim.Length = 0 Then Return 0
   If Not IsNumeric(obj.ToString) Then Return 0
   Return CDec(obj.ToString)
End Function