You can manually 'paint' the grid to suit your needs. Check the options in the AI bar, below the big FULL GENERATE etc. buttons. You should be able to select 'movement' and 'allow' and some such and you'll be able to apply green tiles to the grid using the circle brush.
I know, I was just hoping there was a way to make the automatic AI Grid generator recognize it.
Also, remember to copy the newly created AI folder in your Data\Mods\[your mod] folder into the Levels\[correct level] and overwrite the existing data. If you keep trying to generate the AI and haven't copied it over in awhile (or have quit/rejoined), it will be whatever data was last seen in that level folder.
Unless that problem has been fixed by now, but I don't know if that's true.
I never had that problem. No idea why.
Make sure in the side-war ([X] Is walkable) is checked if that still doesn't do it then maybe those stairs are not meant to be walked upon ;P
"Is Walkable" is checked and the item is used in Luculla Forest. It's the stairs up to the Miners' Office in Silverglen. That apostrophe is probably wrong. Well.