Railworks America

How do i add new addons into train simulator 2012?

Depends on where you got them. You get some that are a RWP file which you can use the utilities program found in your TS directory. You click on the package manager tab to do this. You can also use a program called 7zip to install RWP files. Some come with there own installers and well install into TS. You can also purchase them from Steam and they well automatically install into TS. You say you have TS2012. If you purchase through Steam it well automatically upgrade to TS2015.

Yes and you don't have to install into the .ap files, although routes and DlC may need certain files extracted from there to copy to some repaints etc.
The mods work fine in the proper file directory without being in the .ap archive.