The money was seized by the DHS and probably placed into some sort of trust account (ironically, it may still be sitting at the same bank it was seized from). I don't think DHS ever claimed that was Mt.Gox's money; only that it was money being illegally transmitted by Mt.Gox for US customers.

As such, the DHS should set up a process for Mt.Gox customers to claim the money seized. Mt.Gox no longer possesses the money so you can't sue Mt.Gox to get your money returned (really, what could a judge force them to do?). You MAY be able to sue them for violating the terms of service, not having the proper licenses and thus making you go through the pain of claiming your money from the government but the company is based in Japan so... good luck with that.

Mt.Gox may be willing to refund the money seized -- as someone on this thread claimed happened to him -- but that's completely up to them.