Probably because you don't meet posting requirements.
Anyone submitting a new thread must have had their DIS User name for at least 6 months, and must have a minimum of 50 non-Rent/Trade board posts (that contribute to discussions on the DIS forums) evenly spread over the last 6 months as verified by the DIS search engine. "Evenly spread" is defined as at least 8 posts in at least 4 of the past 6 months.
Yes, but your payment must be processed and the icon (will look similar to the blue Moderator icon under my name) added manually to your account before your thread can be approved. That can take 48 hours, occasionally longer, especially if you pay via e*check, as it must clear your financial institution and then PayPal.
If 48 hours elapses after your payment, and the icon has not been added, please contact on of the Rent/Trade mods. We mods do NOT have access to the payment system, but we can contact WebmasterDoc for you. He has some limited access, but he may need to contact WebmasterCorey. Corey is the only person with full access to the system, and he places the icons.