Well, I´ve gotten the scroll and I now wnat to go to Mind Flayer dungeon to get another part. The problem is that I couldn´t find The Mindflayer key inside Windspear hill dungeon. I now wonder where exactly you get the key, and if there´s any way to use the debug mode (cheating in other words?) thx for answering...

You could use Shadowkeeper or, if you don't want to muck about with that you could just use the basic cheats to summon the key you need. Details on using the cheats can be found at: http://www.sorcerers.net/Games/BG2/cheats.htm
As to which key code you need, I guess it's this one:

KEY29 - Sewer Key

As to were the key is normlaly found in the game, (SPOILER) it is normally found during the Red Dragon killing quest in the Windspear Hills. Though I can't remember if you have to kill the dragon to get it (I think it is just lying about in part of the dungeon).

You get it when you kill Tazok. It lays among the loot from his body.