You have (from what I just checked on your profile) 7 posts. You need 20 (and it may take a few mins for it to kick in after your 20th post).
Here's the officially stated rule (found this in the clearance bin FAQ):
"Users with under 20 posts and who have been registered for less than 5 days will not have access to view or use the clearance bin"
After you've posted your 20 posts, check after a few mins or later in the day on the
main forum page and at the bottom of the list of various folders in the forum, there should be a Clearance bin section with links: "For Sale & Swap" and "Want Ads, Wishlists & CP Requests"
There are some users with over one hundred pigments that sell samples (1165Cheryl and Syrene78 spring to mind).
Hope that helps!