Datastore Location in XenApp 5

This forum thread on citrix.com contains a solution which may work for you:

Thread: Citrix MFCOM Service did not respond as expected

One post points to several Citrix support articles. In one of these articles a possible cause is mentioned:

The Access Management Console in Citrix Presentation Server 4.5 leverages MFCOM and CPSCOM interfaces. In order to use a remote Presentation Server in the Access Management Console discovery process, the remote Presentation Server must be enabled for network COM+ access. If the remote Presentation Server is not running Internet Information Services (IIS), then typically network COM+ access is not enabled.

And a resolution:

Enable network COM+ access. The following describes two methods of enabling network COM+ access.

You will need to configure the web interface Secure Access setting to "alternate" mode, so it actually uses the IP addresses you assigned with ALTADDR.