Assuming you installed properly and connected the antenna wires (there are 2 tiny small connectors that latch onto the 2 round small bnc-like sockets on the 2 top corners of the card), then it might be a problem with the card itself as it's unlikely that an wireless n card will get such slow throughput.
Do the usual checks (making sure your wifi network does put out the speed it should) to eliminate any possibility that the culprit of such slow speeds depends on a faulty router. Once that is certain, then proceed with a process of elimination... if the card is properly installed and recognized then it might be defective.
You could try to return that card and have another one shipped to you to replace and see it was indeed a defective one you received at this time.
Should the issue reoccur on the new card you received, then the problem might lie with the connector on your motherboard. In such case I'd suggest to try and get a wifi card that you could add in one of the free PCIe slots. You can either return the 2nd card and get a return or if the seller where you got those mac pro wifi card sells also PCIe wifi cards then you could get an exchange (in case you got the cards at local store or on ebay).
It's unlikely to have a slow down in speed on that card if other devices get the expected higher speed (which should be from over 54mbps to just below 300Mbps in general).
Hope the above will help you solve your issues... feel free to get in touch should you get stuck.
Best of luck! :)
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