You need to choose a user in order to host the domain. If you don't have a user set up, you can create one while adding the domain within Panel.
[color=#00CC00]Domains > Manage Domains > Add New Domain / Sub-Domain[/color]
[color=#0000CC] Fully Hosted [/color]
Run this domain under the user: <- select user or create one here
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How To Install PHP.INI / ionCube on DreamHost