I thought they had delayed implementing this... which type family were you able to trial?

I sent MyFonts an email and they replied the service is up and running.

I managed to get Brandon Grotesque working on the site, but none of the other fonts I tried including Avenir. However, I copy/pasted the provided code in my css as explained.

Despite having deleted the code for Brandon now, this font is still working, but the other font codes don't.

In case you're interested, I solved it by pasting the code in the original section, instead of going via custom css.