If you can’t watch the livestream at the time it is broadcast, you can take advantage of Niconico’s Timeshift feature that allows you to watch it at your convenience after the live is done. Note: You can only watch it ONCE before it expires.
To reserve a timeshift: Log in (or register if you haven’t). Go to the livestream URL. Click the yellow button on the right with a clock on it (タイムシフト予約). Click the yellow button once more to confirm. Starting from 30 minutes after the end of the performance, you can access your timeshift by going to this page and clicking the red button next to the name of the live.