If you mean where the chat log is stored, you need to go to the directory MxO is installed (default c:\program files\monolith productions\the matrix online\) then open the profiles folder, select the folder with your login name and you will see a file called chat.log which holds your chat log.
Thanks for the assist, jeffers. If you will be acessing the log frequently, while in the folder you may want to right-click on the file and choose Send to->Desktop (create shortcut). This way you have simple double-click access to the file any time.