When you get to the page that asks for the ID's for your 3 selected posts, you will find a link on the right side that allows you to see all of your posts. Right above the link is the number of eligible posts that you made so far this month.
I usually have more than 40 posts eligible before I request the bonus, just to make sure that I'm over the 30 eligible requirement. Actually it's pretty easy to get in the habit of posting and it's fun!.
Select the 3 posts that you like the best: the Id is shown at the top of the post.
The bonus is usually credited towards the end of the month after you requested it. ( request 4/30 is usually credited on 5/30). Since it's a bonus, it doesn't count for the $15 in offers, but does fill in the additional $10 required to get to $25.