Read the last sentence in the bonus rp announcement:
Please note, it may take up to five minutes for the additional Riot Points to be credited to your account and may not appear until you initiate an item purchase in the Store.
I see you have obviously waited over 5 minutes, but have you purchased an item with your rp yet? If not try it, wait a while, and if it still doesn't appear email Riot. I believe they have a team/person dealing with rp transaction problems (correct me if I'm wrong please).
Hope this helps and good luck getting your scrumptious 300 rp!