Radius-TWC has both tweeted the event and posted it on its Facebook page. The event starts at 4 pm ET and will be live. You will need to be registered at Reddit to post a question. Sometime this afternoon shortly before the event starts (from looking at past ones) Keanu or one of his representatives will post an introduction to start things off. Once the thread is established you can start to post questions. It is my understanding that Keanu will chose which questions he wants to answer. Apparently he will see everything that is posted including nasty and/or rude ones. You can favor good questions to move them up the list.
The link is here. If you are interested in participating, check the Reddit site often this afternoon. Also if you need to register, do it sooner rather than later.
Update: I am not familiar with the workings of Reddit. Many thanks to Lelou who has posted the additional info in the comments. If you are a new member at Reddit, you must wait 10 min after posting a question so choose your question carefully.