Using keys in custom fields…? How is it done?

Dear Jordy,

I was very happy to find this list of keys.
In my WP install I’d like to show the “focal Length in 35mm” on the media attachment pages. This ‘key’ is in your list.
I’ve been reading all your stuff the last 3 days and couldn’t figure it out.
How do I go about?

And, if I may, a second question:
Say, in my media files I have custom fields. How do I go about to populate them by WP/LR Sync?

I hope you can help? Thanks so much in advance.
Ron

The page I need help with: [log in to see the link]