Originally Posted by
TheMadBrewer
And I'm rusty, but I think after Z Hd. goes your name, e.g. to have something for me sent to my friend Klaus Schmidt...
Klaus Schmidt
Z. Hd. The Mad Brewer
whereas we would like write
The Mad Brewer
c/o Klaus Schmidt
But then again I could be wrong

You're entirely right.
z. Hd. - "to be placed in the hands of". This would normally be used if you are writing to a company and you want your letter to be given to a specific person. In (British) English this would be "Attn".
If you want to write to Klaus Schmidt who is staying with Hans Meyer you would write
Klaus Schmidt
bei Hans Meyer
(or just "bei Meyer" - using surname only is perfectly acceptable in this context)
"c/o" is generally understood too.
It's important to always include the name of the person someone is staying with (as opposed to just the address) because you could have fifty families living in the same apartment block all sharing an address of "Hauptstr. 2".