Where is my 'junk' folder (upgrade install)...

I've just done a basic upgrade from Tiger 10.4.11 to Leopard 10.5.1. Now the mail app has no junk folder to be found. Junk mail is tagged as junk but just sits in the inbox. Creating a new folder and assigning it as junk mail appears to be an option that is greyed out. My two accounts are standard POP accounts - nothing elaborate or out of the norm.

Anyone heard of this? Anyone have a fix for it?