Click on the "Paid to Post" link at the top of any forum page. This will take you to a page that describes the Forum Posting Bonus.
Basically, you need to post at least 30 meaningful posts in a given calendar month. (I usually have between 40-50 posts before I request the bonus just to make sure I have 30 that meet the acceptable requirements.)
In the middle of the page that describes the Bonus, there is a green link that will take you to the Bonus submission box. On the right side of that page is a count telling you how many eligible posts that you have made so far this month. Right below that is a link to let you see all the posts that you have made.
When you have at least 30 posts, you need to select 3 posts that you like and want to submit. There is a post ID in the title line. Write down the ID's for your 3 "best" posts. Make sure the posts were made in the month you are submitting the bonus request form for.
Go back to the page with the Bonus submission box and enter the 3 ID's for the posts you selected. Confirm that the month shown (in drop down menu) is the month you are submitting the posts for.
Then click submit. If there is a problem, the system will let you know. If all goes well, the system will tell you the submission has been accepted.
If your 30+ posts meet the meaningful requirements, you will receive you $3.00 bonus in your account at/about the end of the month following the month you make the submission.
I try to submit my request on/about the 2nd to the last day of the month.