When performing a behavioral simulation in ModelSim, a warning message similar to the following occurs for the COREGen cores in my design:
"WARNING: xlmult_tb_mult.vhd(261): No default binding for component: "multiplier_virtex2_6_0_9923652091c8ef39". (No entity named "multiplier_virtex2_6_0_9923652091c8ef39" was found)"
You can safely ignore this warning. This warning message occurs because there are two instantiations of the core, one is used for synthesis and the other is used for simulation. ModelSim processes the synthesis instantiation first and issues this warning; after this it processes the simulation model and continues with simulation.