It will work if you tell the code what the values are or the instruction + becomes the default for combining text strings e.g.
Code:
Dim Sum As Long
Dim Sum2 As String
'Sum = Val(TextBox1.value) + Val(TextBox2.value)
'or
Sum = CLng(TextBox1.value) + CLng(TextBox2.value)
Sum2 = TextBox1.value + TextBox2.value
MsgBox Sum & vbCr & Sum2