hello, there's been a few serious issues within the VM installer once the amazon plugin was released. i am going to check it now with opc, but it would be ideal that i check it on your site directly as we do not have amazon account yet.
within opc the only section that handles if the payment button shoudl be visible is the "checkout advertise" section that can be shown/hidden with this checkbox:
this must not be checked for the button to show. you can try to switch opc theme at the display tab in case your theme does not print this secton (all core themes should).
it's important to install VM AIO package either with our unzip.php utility or with a ZIP format of the aio package. There seems to be some serious issues with tar (and gz) format in joomla 3.4.x as it's not capable of unpacking long filepaths that are required by amazon payment plugin.
best regards, stan