Are you using a custom build that you produced yourself from source code or one of my pre-compiled packages? If you made your own build you may have left out some files related to building the search index, these files exist under /Setup/ProviderConfig/indexbuilders
You should also see IndexWriterTask will start about 10-15 seconds later, indexing is not instantaneous, it takes time, it will serialize content into the mp_IndexingQueue table and then the task will process the rows in this table and write to the index, for a large site it can take a little while and if you do anything that recycles the app pool it will kill the task and it cannot finish, so for example you should not manually restart the app or touch Web.config after you start the rebuilding or that will kill it.
Hope it helps,
Joe