There is a parking garage on either side of the Siegel Center. I personally prefer the one on the same side of the street but I guess either would work. If you are coming from the west and traveling east on Broad Street, you will pass by a Lowe's on the left. Some people claim they park in Lowe's lot, which is huge, without any problem, but I would not recommend it.
As soon as you pass that Lowe's, take your first available left turn and that takes you right to the garage. They usually charge anywhere from 8 to 10 bucks, but to me, the peace of mind is worth it. When you come down the elevator out of the garage, it is almost a straight shot to the Siegel Center. Just follow the people. From the time you park your car till you are in your seat should not be more than 5-10 minutes tops.
The other garage is past the Siegel and is diagonally across Broad Street. I guess it works if you don't mind crossing Broad. But I have never had to use that one. I always get in the first one but on rare occasions have managed to find something on a side street. So if you have plenty of time and want to experiment with that, you might get lucky.