greg_zel,
ну так и делайте:
var m = [
['','','','','','','',''],
['','','','','','','',''],
['','','','','','','',''],
['','','','','','','',''],
['','','','','','','',''],
['','','','','','','',''],
['','','','','','','',''],
['','','','','','','','']
]
m[7][7] = 'я последний элемент'
alert(m)