Add the fruit as the fermentation is dying off. Depending on how rapid your fermentation started, should be 48-72 hours after you pitched, but this is highly variable. Considering you will want to let this beer sit on the fruit for a while, you want to consider a secondary conditioning. This is about the only time I even use a secondary. Add your extract to a secondary and then rack your beer from the primary fermentor into the secondary just as fermentation is ending. You will see increased activity in your secondary. If you purchased the extract from your LHBS there is no need to worry about boiling or sterilizing the extract.