Update. Do not use double sided tapes to attach CF spoiler (easy for theft and they will come loose). Instead, use Betalink adhesive or equivalent, which is recommended by BMW M Performance.
If you plan to remove the spoiler one day, then go to your local body shop or professional automotive paint store to buy commercial grade double sided tape. The ones at Home Depot are rated 5-20lbs, not enough to hold a spoiler that generates 150lbs+ of force.
http://www.getbmwparts.com/partlocat...catalogid=4462