Firstly, remove this code in app.lua (although you might want to put in your own banner as backup for when an admob banner is not received).
[lua]
local tt = “images/banner.png”
local i = display.newImageRect(tt, 384, 50) – load and position the in-house banner
i.x = 160; i.y = 455 + yO * 2; i.alpha = 1
bg.banner = i
i.alpha = 1
localGroup:insert(i)
[/lua]
To place the ad at the bottom, try this:
[lua]
ads.show(“banner”, {x=display.screenOriginX, y=display.contentHeight + 10000 })
[/lua]
According to this thread, that should work: http://forums.coronalabs.com/topic/38353-admob-positioning-problem/page-3