What do you mean by "help"? I mean, I got the impression from the news and everything that they were getting lots of help from doctors/police forces, etc, and that maybe civilians would've been in the way. Of course, I completely have no idea what you do for a living, or what your training is.
Look, I'm in Montreal and when it happened and at first they weren't sure who the perpetrators were and what their reasoning was, some of the businesses in big buildings downtown closed for the day. If you think objectively about it, everyone was in shock for the first few days. I was listening to the radio after the first but before the second plane hit, and at that time, they didn't even really know what was going on. They talked about something that happened in Chicago a few years ago, how a small plane hit some building and that was because of fog/weather conditions, and said it was such a clear morning and what was going on? When the planes went down in Pennsylvania and Washington, I think maybe there was a sense that this could happen in other cities, too, or maybe again in the same place.
Basically, don't fault yourself because you have healthy self-preservation instincts. I mean, I have cousins in DC (one could see the smoke from the Pentagon for days afterward, and he's working as a page or whatever for a senator so the anthrax thing was scary for him, too) and that's a freaky thing, to look out your window and see this going on. That's traumatizing. Anyone would be deeply affected by that. Again-- I don't know how close you were, exactly.
You can't go back in time and change what you did--don't let it eat away at you. If you feel really, really, badly about this, try to examine why exactly you didn't go, and try to get around what the obstacle was for you-- fear? selfishness? Were you needed at home for some reason? and think of what you WANT to do next time you see anyone in distress, no matter what the scale of the incident.
Really, please don't beat yourself up about this. The past is the past, and the 11 september thing is hard enough to deal with even without this kind of grief.