Packet Tracer is the simulation software used in CCNA 6.0. Cisco now offers to all NetAcad members (registration is free).
If you are looking for an "offline" experience for the old 6.0 materials, here is the full story of how they were accessed and what your modern alternatives are: 1. The Legacy Path: CCNA Exploration 6.0 Ccna Exploration 6.0 Offline Download