If your loss is allowed in 2007, it must be included in 2007. You must now determine if you have a Net Operating Loss (NOL) and if so, the amount.
To begin you need to complete Schedule A of Form 1045 to determine if you have a net operating loss. Link to form and instructions below.
Once you have determined if you have an NOL and the amount, you may have 2 options. If you have not yet filed the the return, you can choose whether you want to carry the loss back 2 years. First to 2005 and if any amount is left, then to 2006. If there is still some loss remaining, you can carry it forward to 2008. Your other option is to include an election with your 2007 return stating that you want to forego the carryback period and elect to carry the loss forward. If you have already filed the return, you will be required to first carry the loss back, because you did not include the election on your originally filed return.