Some users may not meet their needs in "Photos" app, so they started using "iPhoto" on the Mac. You need to update iPhoto to new versions of OS X with Photos app. Most Mac users will get the error message when trying to launch iPhoto on a Mac with Photos app. The error message they get is: "In order to open "iPhoto.app", you must update to the latest version. The version of iPhoto installed on this Mac is not compatible with OS X Yosemite. Download the latest version for free from the Mac App Store." but when you click on "Search App Store", you’ll get an error saying "Item Not Available". So you need to run iPhoto in new versions of OS X.

Following are the steps to Run iPhoto in New Versions of OS X:

1. Open "App Store" if you get above error.
2. Go to "Purchases" tab of the Mac App Store and locate "iPhoto".
3. Click "Install" located next to iPhoto.
4. This will download and install the newest version which is compatible with OS X 10.10.3+.
5. Go to "iPhoto" app located in "Applications" tab.
6. Drop the iPhoto icon into the OS X Dock for easy future access.

Generally, running iPhoto on a new Mac with Photos app is not recommended. It is best to use Photos app in OS X Yosemite and next versions.

Quick Tip to Keep your Photos Safe
Making backups of your photos & videos is a chore you probably skip, but it really doesn't have to be hard. Here's an effortless way to backup photos - PicBackMan. PicBackMan is the easiest and simplest way to keep your photos safely backed up in one or more online accounts. Simply download PicBackMan (it's free!), register your account, connect to your online store and tell PicBackMan where your photos are - PicBackMan does the rest, automatically. It bulk uploads all photos and keeps looking for new ones and uploads those too. You don't have to ever touch it.