There is no download button to the right.
What do you see exactly?
Just to be sure, you are signed into the AppStore, right ? Your headline say "iTunes Store".
The App Store Application will show you these buttons in the "Purchases" tab to the right - for example
As Butch explained:
- If you want to install it on your iMac, sign into the App Store on your iMac with the AppleID you used when you bought Aperture.
- Click the "Purchases" button. If you do not see Aperture, reload the page ⌘R.
- Install using the "Install" button.
Reasons for Aperture not showing may be a wrong Apple ID or an incompatible System - to download and install Aperture on your iMac you need at least MacOS X 10.7.5 on your iMac.