@ firas hentati: There are different ways to use multiple Lua files as you can see.
You do not have to change mp_luaserver at all when putting the files into sys/lua/autorun as suggested by Starkkz. It's actually the easiest solution.
You could also keep mp_luaserver as it is (the default value is sys/lua/server.lua) and put the stuff that Xirot wrote in your sys/lua/server.lua file. Don't forget to change the paths/file names then.
This is actually the classic solution (because the autorun feature is new and has just been added recently for convenience)
Note that specifying MULTIPLE Lua files directly in mp_luaserver does NOT work. So this setting is not really related to your problem.