I would make a small change in the way you write your conditions so as to remove some redundant duplication of code.
If Index = 7 Then
MsgBox("Maximum amount of data entered, reset to add more.")
ElseIf txtShoeSize.Text = "" Then
MsgBox("Enter a Number.")
Else
If Num >= Maximum Then
Maximum = Num
End If
NumArray.Items.Add(txtNum.Text)
Index = Index + 1
End If
If you provide a little more information as to the "does not function properly", that would help us help you more. Describe to us what actually happens versus what should happen.