I do not believe that the AMOS add-on is available for Mac OS, only for Windows operating systems (see: https://www-01.ibm.com/support/docview.wss?uid=swg21618307). Therefore, your only recourse would be to load a Windows-permitting app on your Mac, such as parallels (parallels.com), then install Windows, then install IBM SPSS, then install AMOS.
The AMOS program is both copyright and not freeware, so there is no legal way to download it from the internet, unless you happen to work for an organization that has a site license for IBM SPSS. A number of universities have site licenses that allow their students to load SPSS and any add-ons covered by the license. If this is not the case for you, you might investigate leasing a student version of SPSS at a site such as onthehub.com For students, a 12-month lease is very reasonable in cost.