I don't think you can install webmail apps on a shared server. That should require root privilege.
However, you may be able to install on PS. It's better to ask DH support.
And if you are not familiar on how to manage webmail apps, it's better to use DH's or Google's one. They have professionals to manage that.
$50 off and 3 free domains with code: [color=#CC0000]DH3[/color] [url=http://dreamhost.com/r.cgi?296804/signup|DH3]Sign Up NOW[/url] or More Codes Here