text Missions_txt
{
     strings = 3; //Number of Levels +1 (3 for are 2 possible Levels)
     string = "";
              "Level1.wmb"; //Names of the Levelfiles (this will be loaded for 1)
              "Level2.wmb"; //this for 2
}

This is the action with its event to load the new level:

function LevelLoad_event()
{
     my.enable_impact = off;

     var LevelNum;
     LevelNum = my.skill1;

     my = NULL;
     you = NULL;

     level_load(Missions_txt[LevelNum]);
     wait(3);
}

//skill1: LevelNumber 1
action Lvl_Change_Run()
{
     my.enable_impact = on;
     my.event = LevelLoad_event;

     my.polygon = on;
     my.fat = off;
     my.narrow = off;

     my.invisible = on;
}
}