[Relaxing]

You forgot to complie the script file,
upload it to your plugin dir.
------
For lazy admins:

register_clcmd("amx_give_xp", "clcmd_xp", ADMIN_LEVEL_A, " - <name> <xp>");

public clcmd_xp(id, level, cid){
    if(!cmd_access(id, level, cid, 3))
        return PLUGIN_HANDLED;

    new arg[32], arg2[32];
    read_argv(1, arg, charsmax(arg));
    read_argv(2, arg2, charsmax(arg2));
    
    new player = cmd_target(id, arg, 2);
    if(!player) return PLUGIN_HANDLED;
    
    new xp_amount = str_to_num(arg2);
    xp[player] += xp_amount;
    
    if(xp[player] < 0) xp[player] = 0;
    
    new player_name[32], admin_name[32];
    get_user_name(player, player_name, 31);
    get_user_name(id, admin_name, 31);
    
    switch(get_cvar_num("amx_show_activity")){
        case 2: client_print(0, print_chat, "ADMIN %s: gave %s %i XP", admin_name, player_name, xp_amount);
        case 1: client_print(0, print_chat, "ADMIN: gave %s %i XP", player_name, xp_amount);
    }
    
    return PLUGIN_HANDLED;
}