Did you try copying the disc to your HD first and then installing? Alot of times you can get past disc errors that way. Been awhile but I can't recall whether or not SE has an expansion download service.
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People don't like to be meddled with. We tell them what to do, what to think, don't run, don't walk. We're in their homes and in their heads and we haven't the right. We're meddlesome. ~River Tam
Sedao