The Raid Leader's text is a very pale color and it may be very difficult to see, as Tandaa has suggested. If you have tried changing the color of the text and are still unable to see the your Raid Leader's messages, it may be caused by an issue with your user interface (UI).
First, please try disabling any custom UI addons that you have installed. If you are still unable to see the text after doing so, I would recommend resetting your UI by following these steps:
1) Log out of World of Warcraft to your desktop, thereby exiting the game completely.
Please let us know if this helps, Sitagahze.
2) Navigate to the World of Warcraft folder, which is often found in the Program Files directory (the default path is C:\Program Files\World of Warcraft).
3) Either rename (or delete) the WTF, WDB, and Interface folders. (Note: If you do not wish to permanently lose any third-party addons you may have accrued, or macros you've written for your characters, you would be well-advised to merely rename the folders, rather than delete them entirely.)
4) Log back into the game by double-clicking WoW.exe, ensuring that you do not reinstall any addons before doing so.
There's someone in my head, and its not me...