How to use WP Toolset Custom Fields and Taxonomy Variables in Yoast SEO.
Its very easy:
For Custom Fields you use the field slug combined to that string:
%%cf_wpcf-modell%%
In my case modell is the custom field slug.
For Taxonomy variables you can use this:
%%ct_hersteller%%
with hersteller as the taxonomy.
You can check if its working when you have a look at the source code snippet at the meta title and meta description.