There is your problem, you have posted in a 2008 forum, so you got a 2008 answer, when you should of posted in the 2005 forum, or detailed from the start it was 2005.

The only way to rebuild a heap in 2005 is either pump data into a new table or create a clustered index then drop it, but would seriously recommend keeping the clustered index.

http://sqlblog.com/blogs/louis_davidson/archive/2008/02/26/2008-rebuilding-a-heap.aspx