I have 10.6.8 and cannot get Mavericks to install

ugh. Please assist.

I have 10.6.8

my imac came installed with 10.6.2 as a "Mac OS X update combined" according to my software update log (21/12/09)

it updated to 10.6.8 on 16/07/11

I go into the APP store and, as directed, hit download for Mavericks and it just sits there after stating that it is downloading - for hours - repeatedly.

nothing!

please help.

Thank you,

LynaLovely