Do you mean that you spelled the name incorrectly, or perhaps you registered the domain with the wrong TLD (for example, .net instead of .com)?
If the latter, then it may pay to register both anyway and redirect them to the same web-site.
I doubt you can change it if you have already received the domain registration confirmation email. However, if you have not yet received this email, then there may be a small chance that DreamHost can change it for you. Contact them as soon as possible to find out.
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