Sorrry My OP was VERY unclear.

I also cannot find it in the WoW Client interface.

It is available through scripts

so /script SetBinding("CTRL-F1","/cast Howl of Terror");
Would cast Howl of Terror when you press the CTRL-F1
You also have to explicitly call
/script SaveBindings
(or edit another key using the client)
If you now exit wow you can see in the bindings key that it has added a line for your entry
http://www.wowwiki.com/API_SetBinding

There are similar api calls for SetMacro, EditMacro and EditBinding etc...

For me, I didnt want to install an addon just for this so I was pleased to find an easy way to do it.

Apologies again if I sent people off on a wild goose chaase