In reply to jnbkeller:
A simulation can have many top level modules. Modules have initial and always blocks running as concurrent processes; it does not matter what the hierarchical relationship between the modules is.
Tools have different compilation flows to figure out which are the top-level modules. This Mentor/Siemens EDA sponsored public forum is not for discussing tool specific usage or issues. Please read your tool’s user manual (I’m sure the info is there somewhere) or contact your tool vendor directly for support.