Using BUFGMUX component in Spartan-3

Has anyone had any experience directly implementing BUFGMUX components in a 
Spartan-3 device.  I know this sounds like a very simple task but I can't 
seem to get this thing to route.  I am simply attempting to select between 
2 separate clock signals and route them to a DCM to be deskewed prior to 
use throughout the device.  The part is a Spartan-3 XC3S400-fg456c -4.  The 
2 clock signals are on the same side of the die and connected to GCLK-5&7 
pins respectively.  When I run this through Synplify Pro v8.1 and then 
through ISE7.1.2 I get a warning stating that the output pin of the BUFGMUX 
is not connected to a external net, and the tool adds the net / pad for me.  
Then when it moves on to build the DCM it fails do to the fact that the 
clkin pin of the DCM has multiple drivers the BUFGMUX and the pad that the 
tool added.