changing a select box value with js

[Carnage]

ok what i've got is a page with an image on it, it has an impage map and clickable areas on it, what i'm wanting is for whenever an area is clicked it to change the value of a select box to something else.

i tried: onmousedown='document.forms[6].selectbox.value=newvalue

it came up with the error: forms[6] has no properties

i also tried using the forms name in place of forms[6] same error... any ideas as to what this error means and how to fix it?