Well Proteus may be great at modeling embedded microcontroller systems with mixed analog and digital components, but it's not a functional and timing logic simulator like Modelsim. The two tools perform different functions, which is clear after looking over the labcenter web site for a couple of minutes.
Seems to me like you're trying to use a pair of pliers to tighten a hex head bolt...about all that does is round off the bolt and makes you frustrated.
Regards