To reach hell you must first reach the City of Gold. Follow the guide in City of Gold to reach it. When you're there, look for a book known as the Necronomicon. Pick that up and proceed to the fight against Olmec. One you get here find the door under the terrain and choose that as the location to kill Olmec. Have him drop over it and use his head before he dies to enter the door. There are a lot of new enemies and traps in Hell, plus a whole new boss. It is a very difficult level to get through and will require some knowledge that I'd rather not try to sum up in words. Watch the video for help.