You need to update to Aperture 3.6, but since Apple stopped selling Aperture you can neither update nor purchase Aperture, you can only reinstall.
See this User Tip:
Updating to Aperture 3.6 After the Release of Photos
Only Aperture 3.6 is fully compatible with MacOS X 10.10.3, so you need to update to version Aperture 3.6. after upgrading to MacOS X 10.10.3.
But Aperture is no longer sold at the Mac App Store, and the Updates page will show "Currently not available", and we can neither update from the Updates page nor buy the upgrade from the main "Featured" page of the store.
As a work-around: Reinstall Aperture instead of updating it.
- If Aperture is associated with your AppleID as you bought it originally from the Mac App Store or you accepted a boxed Retail version to your AppleID when you updated to Aperture 3.4.5 or later, you can reinstall from your Purchases page. In all other cases you need the help of the App Store Support as described in the last paragraph.
- Open the Purchases tab of the AppStore and sign into the Store menu with your AppleID. Is Aperture listed among your Purchases? It may be hidden. Try to unhide it, see this link: http://support.apple.com/en-us/HT4928
- Then proceed as described in Barney-15E's User Tip for iPhoto: (I replaced "iPhoto" by "Aperture" in the quote)
Get iPhoto 9.6.1 if you didn't update before OS X 10.10.3
Just as a precaution, copy Aperture from your Applications folder onto an external disk or USB stick and eject that drive.
Delete Aperture from your Applications folder. Make sure there are no disks mounted that have a copy of Aperture on them. Empty the Trash.
Make sure no other users have Aperture open in the background. It is best to have all other users logged out.
I ended up having to Restart before I could clear Aperture from the App Store memory (it was still showing Update on the button in Purchases).
If it is still showing as Update, try searching Spotlight for Aperture. There may be a copy on a mounted external disk that it is identifying.
You should now be able to go to the Purchases tab and find Aperture with the Update button labeled "Install". Click "Install" and it should download the current version which works with Yosemite.
When I first tried, after authenticating, it posted a 503 error and asked if I wanted to Retry. When I clicked "Retry," it started downloading.
It's about 1.01 GB, so it takes a little while to fully download.
2. If your Aperture 3 version does not show at the Mac App Store Purchases page, give the App Store Support a call. There is a "Support" link in the Quick Links on the main page of the Mac App Store. Ask for a redeem code to be able to download the update. You will probably need to provide a proof of purchase and be very insistent.
If that does not work - but it should - , post back. You can force Aperture 3.5.1 to run on Yosemite.
Open the Aperture Application package by selecting Aperture in the Applications folder. Ctrl-click it and use "Show Package contents".
In the package open the folder Contents and then MacOS. Drag the "Aperture" item to the Dock and click it to open.
