Calling a plpgsql function using a trigger in pgadmin III

Hi folks:

I'm very new to postgresql/pgadmin and having some trouble. Let me try to
explain what I'm doing:

I have a function under 'Functions' which takes one text argument. I want to
add a trigger in one of my tables to call it.

Under that table I right-click Triggers and click Create, and I suppose this
is the point where I'm lost.

In the properties window that comes up there's a dropdown box for a trigger
function, and it's empty. So I create a new function under the 'Trigger
functions' branch, and the dropdown box is still empty.

I'm sure this is a real newbie question. Can anyone give me some guidance?

Thanks.