Creating a MAC

[Heather Shaw]

Hello,
  I am a newbie so please forgive stupid questions
but, I am trying to create a message digest using the
gcry_md_open (&md, GCRY_MD_SHA1, GCRY_MD_FLAG_HMAC). 
In the documentation it states that you must call the
gcry_md_setkey to be used to set the MAC key.  How do
I calculate this MAC key?  What is the MAC key...does
it get generated from the message itself in
combination with another key of some sort. 

Thanks
Heather