Although you have reached 30 posts, you still have a few more days to get a few more posts in.
Whether you submit your request on the 5th, 15 or 27th of a month, the bonus will be credited on or about the 31st of the month following the month you made the request.
I usually wait until the 2nd to the last day of each month -- and I usually have 40-50 posts to select my 3 "best" posts. It's safer to have more than 30 just in case some of the posts are deemed not meaningful. (Posts made to the Contest and Promotion section do not count -- figure you are already getting a bonus for them).
When you are ready to select your best 3 posts, click on the link "paid to post" at the top of any forum page. This will take you to the page that explains the paid to post bonus. In the middle of that page is a green link that will take you to page with the form you need to complete your bonus request.
There is a box asking for the ID's of your 3 "best" posts on the left side of the page. On the right side of the page is a count of the number of posts you've made so far this month and right below that is a link for you to see all of your posts. Click on that link to look through your posts and select the 3 that you like and write down (carefully) the ID #'s of those 3 posts. Make sure that the posts you selected are all in the current month.
At the bottom of this page is a history of all the posting bonuses you've made/received. You can check this section to see that status of your bonus.
Return to the previous page and enter the 3 ID's in the box provided. Then submit.