Same as YKT above, I watched that Mayu live on timeshift. Try creating an account and logging in. Please note there are limited amounts of slots available on nico-live so not everyone who wants to will be able to watch, and you can "reserve a timeshift".
Clicking the link YKT provided seems to suggest that Premium (ie. paid membership) will have higher quality, and obviously, those members will be prioritised in getting the slots to view the stream.
For the Mayu event, you could view the livestream from any region - but I have heard of other Nico livestreams where you have to have a Japanese IP to view it - I do not know the case for this stream.
I assume that DMM.com will be livestreaming the theatre performance like they do all theatre performances for AKB48 Monthly Subscribers only. A months subscription is 3000 yen, you may have an issue paying with foreign credit cards and the LODs are IP-locked to Japan.