There's no Wishpicks app to download from the App Store or Google Play. What you can do instead is add the Wishpicks website to your phone's home screen from your browser. It then opens full-screen, like an app.
Steps
Open wishpicks.com in your phone's browser.
Use your browser's own menu to add it to your home screen:
iPhone (Safari): tap Share, then Add to Home Screen.
Android (Chrome): tap the menu (three dots), then Add to Home screen or Install app.
Tap the new Wishpicks icon to open it full-screen.
For more detailed steps, open wishpicks.com/features/mobile on your phone.
Good to know
There's nothing to download. The home-screen icon is basically a bookmark that opens Wishpicks full-screen, so it behaves like an app without an app-store install. Wishpicks won't nag you to add it, either. There's no in-app "Install" button; it's a manual step in your browser.
You'll need to be online. This isn't an offline app.
Everything works the same as on the web: same sign-in, same features. See "Do I need an account to use Wishpicks?".