Of course you'll need to be using Chrome on your Mac or PC, but the app will run the same nonetheless.
- First you'll need to open a tab to the Google+ Photos app in the Chrome Web store.
- You'll notice the "Add to Chrome" button is grayed out. Right-click on it and select "Inspect Element" from the drop-down menu. A text editor will open on the lower half of your screen with this element already selected.
- Press Command-F (on Mac) or Control-F (on Windows) to search for "webstore-button-disabled" (without the quotes). The searched text will then be highlighted and you'll need to double-click on the phrase to make it active. Delete the text and press Enter.
- The Install button will turn blue and you'll be able to click it to install the Google+ Photos app. The first time you run the app you'll see a prompt alerting you that you're running it on an unsupported operating system; click on Run anyway and log in to your Google account.
You can then set the app to automatically upload your imported photos from your computer to your Google+ account, or use it as a standalone photo viewer. If you run into trouble launching the app after installation, restart Chrome and try it again.