The ARA MP Server v2.1 that ships with the Workgroup Server 7250 and 8550 works only with the built-in enabler on those computers. Only ARA MP Server v2.1.1 supports PCI cards.
There is no upgrade from the bundled version (2.1) of ARA MP Server to 2.1.1. If you wish to use ARA MP Server with a PCI card, you should purchase version 2.1.1.