When we install for the first time an extension that adds a menu on the System » Configuration panel, it is possible that we receive a 404 error when trying to configure it.
This has happened to me on the Simple Configurable Products extension and the AheadWorks Blog extension. Solving this is really easy:
1. Clean your cache and reindex data
2. Log out from the backoffice and log in again
Everything should work fine now.