Syeed,
This is based on V6, so the explanation may be slightly different fro V7 if that is what you are using.
I would read the last few pages of the API manual first, which is found in the programming folder of the Aimsun installation. Section 4 and 7 explain the process. The manual recommends VS 2005 or 2008. I just tried the basic function call skeleton contained in the AAPI.cxx file and it compiled in VS2010. I generally use python to write API's so I have limited experience with C++.
Here is how I did it. In visual studio, create a new win32 project called AAPI. Before the project is created, the VS wizard asks what type of application to create, select DLL. On the same page there is an additional option to create an empty project, select this as well.
Copy all of the files from the C:Program Files (x86)TSS-Transport Simulation SystemsAimsun 6programmingAAPI61AAPIVisual folder with a .h, .lib and .cxx extension. Paste them into your newly created project folder and then add them to the project. Build the project (not run) and then within the debug folder there should be a file called AAPI.dll. You add this to Aimsun within the "Aimsun API" tab of a scenario