Q: I have downloaded OS X Yosemite and cannot open iPhoto the message comes up that this Mac is not compatible with OS X Yosemite and to download the latest for free from the App store but on going to the app store it states that the item requested is not cu

Mac OS X 10.10.3 and newer include Photos. If you'd like to go on using iPhoto in addition to or instead of switching to Photos, click here and try following the instructions.

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I am confused because you seem to be asking about at least two separate problems.

Are you unable to download Yosemite from the Mac App Store?

What OS X version is installed on your iMac at present?

iPhoto has been replaced by Photos, but iPhoto still runs in Yosemite. If you cannot update it, you may find Barney-15E's User Tip to be applicable to your concern: Can't Update iPhoto because it is not available in the App Store