Adding attach download link to Threadbit

[Powlo]

I'm customisizing my threadbit and i want a download button for the attachment contained within the thread but im having difficulty getting the url correct.

Ive been using this ..

<a href="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]&d=$attachment[dateline]">

but that doesnt work.

Any ideas where im going wrong?