[Dayrion]

Your indentation is completly wrong.
Like MotherDucker said, define your dialogid before use it.

PHP код:

#define FIRSTMENU    990
#define SECONDMENU  991
public OnDialogResponse(playerid, dialogid, response, listitem, inputtext[])
{
    if(dialogid == FIRSTMENU)
    {
        if(reponse)
        {
            if(listitem == 0)
            {
                ShowPlayerDialog(playerid, SECONDMENU, DIALOG_STYLE_LIST,
                "Regular Players Lounge",
                "MP5\nCombat Shotgun\nDesert Eagle\nM4\nAK-47\nBack",
                "Select", "Exit");
            }
        }
    }
    
    if(dialogid == SECONDMENU)
    {
        if(reponse)
        {
            switch(listitem)
              {
                case 0:
                {
                    new ammo = GetPlayerAmmo(playerid);
                    if(ammo >= 600) return SendClientMessage(playerid, COLOR_GREY, "Sorry you have all the free ammo we can give you for now");
                    if(GetPlayerMoney(playerid) < 15000) return SendClientMessage(playerid, COLOR_GREY,"* You cannot afford this!");
                    GivePlayerMoney(playerid, -15000);
                    GivePlayerWeapon(playerid, 29, 200);
                    return 1;
                }
                case 1:
                {
                    new ammo = GetPlayerAmmo(playerid);
                    if(ammo > 400) return SendClientMessage(playerid, COLOR_GREY, "Sorry you have all the free ammo we can give you for now");
                    if(GetPlayerMoney(playerid) < 8000) return SendClientMessage(playerid, COLOR_GREY,"* You cannot afford this!");
                    GivePlayerWeapon(playerid,25 ,25);
                    GivePlayerMoney(playerid, -8000);
                    return 1;
                }
                default : return 1;
            }
               return 1;
        }
    }
     return 1;
} 


pastbin with right indentation: http://pastebin.com/n6vPZuHC