You need DYOM mod for the missions to work.
Installation-
1.Download DYOM mod from GTA Garage.
2.After installing it,copy these .dat files in the DYOM folder.

Play-
1.Start a new game.
2.Select DYOM
3.After the game starts open the menu by pressing 'y'.
4.Go to mission menu.
5.Select load mission.
6.You will see the list of the missions just like save games.
7.Load them.
8.Go to mission menu again.
9.Select 'Play Mission'