I had the same problem. I am using Add-Meta-Tags as well as The Events Calendar on my site. I wanted AMT to add “NOINDEX, FOLLOW” robots meta data to events categories as well as normal blog categories.
I came up with the following solution: in the function amt_get_metadata_head() in add-meta-tags.php, add the is_tax line in the following section of code:
( is_search() && ($options["noindex_search_results"] == "1") ) || // Search results
( is_date() && ($options["noindex_date_archives"] == "1") ) || // Date and time archives
( is_category() && is_paged() && ($options["noindex_category_archives"] == "1") ) || // Category archives (except 1st page)
( is_tax('tribe_events_cat') && ($options["noindex_category_archives"] == "1") ) || // Hopefully cope with event category archives
( is_tag() && is_paged() && ($options["noindex_tag_archives"] == "1") ) || // Tag archives (except 1st page)
( is_author() && is_paged() && ($options["noindex_author_archives"] == "1") ) // Author archives (except 1st page)
I realise that this will get overwritten if I update the plugin but for now it works for me. What AMT really needs is somewhere in the control panel where you can list the names of your custom categories so it automatically includes them as well.
Hope that helps somehow.
Cheers,
John.