On the Gifts tab in the Post to General Ledger parameters, click Include and select 'All Gifts' or 'Adjustments and Reversals only'.

One thing to note is that reversals post to the date of the original gift. Therefore if the original gift has a post date of 12/1/10, but then the gift was deleted on 4/15/11, the reversal is trying to post to the original gift date. So, if you are including a date range when posting gifts, it is possible that the reversal may be excluded. Also, when running a pre-post report or posting, if including a gift query, adjustments and reversals do not pull in. The Post to General Ledger settings on the Gifts tab must be set to either All Gifts or Adjustments and Reversals only.