The tickets in the Lawson machine (called "Loppi" btw) sells out like superfast. Once i stood ready 5 minutes before the Loppi machine even would start and on the minute it booted up at 10:00 I went in, found ayu's concert one minute later and when I was going to book my tickets they where already sold out. It's a reaaally hard way to get the tickets one.
If I was you I would either beg a nice japanese friend to buy a ticket trough Yahoo! Auctions.jp (since you pretty much must live in the country to register there) or go to a reseller ticket store when you arrive to Tokyo.
My favourite option is actually a reseller store. You can choose seating of your liking (and can afford to) and get the ticket right in your hand. And I'm pretty sure tickets will be avaible when you arrive. I once bought a CDL ticket one hour before the show started (I did that cuz when the concert starts they just want to sell off the remaining tickets really fast so they lower their prices). Anywho, that is what I would have done.
Good luck!