I see from the website that HighPoint seem to produce separate cards for PC (RocketU 1022A and 1144A) and Mac (RocketU Dual for Mac and RocketU Quad for Mac). You have the Mac-specific model, right? If so, there are drivers on the website, which should've come on a CD with the card but you never can tell these days. Are those installed and/or updated? I see the current driver revision is 1.0.1. Support is listed for OS 10.6 or 10.7 only. The drivers are available inside the "Download Center" tab of each product page on "http://www.hptmac.com/US/categories.php?compatibility=18".
If all that's OK, I don't quite know what else to suggest, having no experience with these exact cards. With the PCIe slots available inside your computer, you can't really have gone wrong but it's worth trying the other x4 slot if you can.
If all else fails and support stays as awfully unhelpful, there are alternatives. The one I've installed and verified as working is the LaCie USB3.0 card. It requires Leopard or later and a driver install but seems to work just fine. And LaCie have more people you can call to complain to if need be.