For loyalty rewards members, ARP boost options are now available to claim in the rewards section.
After claiming a boost, you can activate it in the My Boosts section in your account settings. You can only activate one at a time, so choose wisely...
As a note for all the non-US members who are sure to become enraged that they are being excluded, I will reiterate what was mentioned when Arena Rewards launched. We haven't forgotten about you. This program is rolling out small at the start. Once we have everything worked out, we want to expand to the rest of you. So continue to level up and when we finally launch in your country, you'll be able to claim all the rewards. (pro tip: you can claim rewards retroactively)