If you are trying to add an item to your promotion and the item is not appearing in the search then the item will already belong to another promotion.
The same item cannot exist in two different promotions. If the item exists in a promotion that is no longer active it must be deleted from the "Items Selected" section within the inactive promotion before you will be able to add it to the new promotion.
A quick way to find if the item exists in another promotion is to use the "Print Promotions" option, include inactive.
You can then use the find function (CTRL + F) to search for the stock code of the item
You will now see if the item already exists in another promotion.
If the promotion has already ended or is set to inactive, edit the promotion, select all delete selected.
The promotion should now look like this
Go back to the promotion you are trying to set up and try add the item, it should now show.