Well when using as a regular custom island. If you have a landscape banner.
It will push everything over to the right.
So keep the island but do not show it.
Then goto:
Control Panel ยป Forum Settings
Select the forum details for the forum in question.
Then select the tab"Custom inserts"
In that area you will see:
Custom Island Insert
Select the island and will appear in the middle of the page as noted.
"This will show up between the 1st and 2nd post of a topic. Usually the most effective place to put ads."
Or you will need a banner that is in portrait mode to show up on the side.
Also it would be helpful if you post the url to the forum so we can view it.