xilinx's black box external co-simulator problem with ModelSim

Hi I am using matlab 2012a + xilinx ise with system generator 14.1 + Modelsim SE 10.1c. I am using win 7 64bit and so with above tools are all 64bit version. I am simulating the black box from xilinx in simulink using ModelSim. The simulation works fine initially, but after a while, an error pop up stating "The helper block required by the external co-simulator is missing. Please place the appropriate helper block with name ModelSim in the same level of heirarchy as this block." I have confirm that I have put the ModelSim block together with the black boxes, and the 'HDL co-simulator to use' section I put ModelSim. What could be the reason of this error, and how to fix it. Thank you.

[Kaustubha Govind]

Charles: Since you are using System Generator (a Xilinx product), you might have better luck getting help from Xilinx Support.