(first let me say, Taka... reboot your car! It will work!! Take the negative battery terminal off for 10 seconds and replace it. your trunk release doesn't work either... been there, fixed that!!)
On the battery replacement problem, here ya' go....
Take out the screw... (duh!)
Then while slightly prying the lid up next to the button, slide a small flat head screwdriver tip into the slot you just created... do it just on the other side of the key post. Then gently slide, do not pry, the screwdriver tip around in the crack till you reach the top of the key. It will pop right open.
The lid attaches via the screw and a flap at the very top on the bottom side of the opening for the key fob. I wish I had my digital camera here at work I would post a picture.
The battery is a 3 volt battery. Any 3 volt of the same size will work. It is a 1216 battery. Energizer CR1215 is what I am currently using. Make certain that when you take out or replace the battery you be careful not to break off the small metal clip that grips the side of the battery. You should slid the battery in and out on the opposite side of this clip. Make sure when you install the new battery, you slid it under that clip.
Well there you have it, more information than you ever wanted to know, in boring, painstaking detail!!
gotta' love a Lexus!!
(did I mention my wife's name is Alexis??)