"Alert: os x mavericks cannot be installed on this computer"

Hey guys- I've seen a few of these threads, and none of the answers have worked for me, and I couldn't find a general reason or solution. I'm trying to Install mavericks, and as soon as I click on the installer, I instantly get the above error messge-- ""Alert: os x mavericks cannot be installed on this computer".

I can't for the life of me figure out why.

My specs:

15" Mid 2009, macbook pro

10.6.8

4gb Memory

over 100g free space

2.53 ghz intel core 2 duo

No available updates. Would really appreciate any insight you guys have, or any possible solutions you could think of.

Thanks a million,

John