The source code is proprietary to Visual Numerics and is not available. I'd suggest asking at the VNI IMSL forum where the VNI engineers and scientists hang out, but I see you've already done that.
IMSL
may let have you access to the source code, but at a steep price.
The Complex method is effectively the Simplex (Nelder-Mead) method with support for bounds. (It was a deliberate play on words.)
I see that your IMSL posting has given you suitable references & C++ source code. I'd be inclined to go to http://plato.asu.edu/sub/nlores.html and use the COBYLA codes as being the best equivalent.