You probably would have received a response sooner if this was asked on the Excel Questions board, but at any rate here's one way to do what you want:
Dim AckTime As Integer, InfoBox As Object
Set InfoBox = CreateObject("WScript.Shell")
AckTime = 3
Select Case InfoBox.Popup("Click OK or do nothing within 3 seconds.", _
AckTime, "This is your Message Box", 0)
Case 1, -1
Keep in mind that the first time code compiles, it can take a few extra moments, so on the first execution of this macro, it will take longer than 3 seconds for 3 seconds to elapse. Run the code twice and see if the second time around is closer to 3 seconds than the first execution, which it was for me.