In order to run AECOsim functionality within Generative Components you will need to download and install the Generative Components Extension, not the standalone version.
During the installation you will be prompted to select which application you would like to apply the Extension. Select AECOsim Building Designer, and then Next.
Once the install is complete launch Generative Components and you should find Building Designer within the pull-down.
NOTE: BIM integration for older versions of Generative Components would require the use of the “BIM Features for GC Extension”, in which Part and Families are made available with use of DLL files. More on this can be found at: http://communities.bentley.com/products/products_generativecomponents/w/generative_components_community_wiki/bim.aspx