I'm not very familiar with Excel but managed to construct this:
Sub DDUOM_Change()
' DDUOM_Change Macro
' Macro recorded 10/22/2009 by jlute
If Range("B5") <= 4 Then
Range("C5") = Null
End If

If Range("B5") 4 Then
MsgBox "Density is required!"
End If

End Sub

I want the cursor to go to cell C5 after clearing the message box
however I can't seem to work that out. I know GoTo is the right
command but I keep sending the debugger into fits.

Also, once the cursor is in C5 and If B5 is 4 I don't want the user
to be able to exit C5 until a value is entered. How can I do that?

Thanks for your help!!!