Since RTW the Editor tool has been a complicated affair and it remains to be seen if the edit you made to the preferences.cfg file remains after the patch (pre patch it reverted after a reboot)
OK, so once you open the editor, you can decide on a location on the campaign map and this will open that particular map location. Along the bottom of the screen are a list of icons and you will need to select your settings for the proposed battle.
(I am trying to recall from memory here as I do not have M2TW but it is the same idea as RTW so np except I am starting to succumb to the influence of wine)
Right, you need to choose your settings. The factions, who is allied to whom and the victory conditions. You then get to select the general commanding each faction army and his rating. Once you do this for each faction you will enable the deployment icon.
Now you are able to place the deployment zones of each army and deploy individual units,choosing their size etc. Once this is done you save the battle conditions.
The battle_descr file is editable so you can change unit sizes, stats etc until you are happy.
The editor tool is long winded but well worth the effort. I hope this helps you, if not feel free to PM me and I will try to talk you through it, it seems very similar to RTW so I should be able to help with any problem you come across

.....Orda