GI,MSGBOX,'Window Title','Your message',OK

will give you a message box with "Window Title" in the title bar "Your message" in the message box and an OK button.
Make sure Title and message are enclosed in ticks (')

From WinCMES2.0 release notes
For displaying a common dialog box & getting user response:

**GI,MSGBOX,HeaderText,MessageText,Buttons,Icon

Buttons options are:

ABORTRETRYIGNORE / RETRYCANCEL / OKCANCEL / OK / YESNO / YESNOCANCEL

Icon options are:

INFO / EXCL / STOP / QUES

User response is put into $GI global symbol.

Duncan