It's implemented in VFP 7 only. You were looking into VFP 7 help in MSDN.

>MSDN told this!!!
>
>Displays a user-defined dialog box.
>
>MESSAGEBOX(eMessageText [, cTitleBarText][, nDialogBoxType ][, nTimeout])
>
>nTimeout
>Specifies the number of milliseconds Visual FoxPro displays cMessageText without input from the keyboard or the mouse before clearing cMessageText. Any valid timeout can be specified. A value of less than 1 will never timeout until user input (same as without the nTimeout parameter). When a timeout occurs, MESSAGEBOX( ) returns –1.
>
>
>when i tried to use parameter timeout,
>sample : MESSAGEBOX("HELLO",36,"MyTitle",10)
>
>i got the following error message:
>too many arguments
>
>i use VFP 6.0
>
>thanks for any helps!!!
>Martin