Answer:
1- Download and install the CLEO 3 Library (HTTP://cleo.sannybuilder.com).
2- Download the DYOM v5 (HTTP://dyom.gtagames.nl)
3- Open the zip file with WinRAR or anything like that, and extract the MPACK folder
to My documents\GTA San Andreas User Files
4- Run the game.
5- Click on New Game, and then on Design Your Own Mission.
6- Create your Mission!