Unable to add (see) Featured Image in CustomPress-created; unable to create no menu item

I haven't been able to integrate the Featured Image (thumbnail) in the custom post type that I've created. That is, it isn't in the post layout among the right sidebar settings.

It is set up in the post-type settings, with this description:
" current theme must also support post-thumbnails. "

In the single.php for my theme I find:

$image_id = get_post_thumbnail_id($post->ID);

How can I make sure that my theme supports post-thumbnails correctly? In which file would I look?

Also - I am unable to create a menu item for the Custom Post page - if I use the title I created, it generates an error.

Thanks in advance for your help!