I don't know if you solved the problem mate, but I think OPNET is not very optimized with Visual Studio 200x in 64 bit architectures. I personally experienced alot of issues setting up the simulation infrastructure, and I just was failing with getting it done, and the time was passing by. When I was exhausted of trying, I created a virtual machine, 32 bit OS, with 32 bit Visual Studio 2008, installed the OPNET 14.5, added the system environmental variables, and finally, a successful simulation. Hope you did your work. Cheers mate.
Redon.