I downloaded picasa web album hd app on my MacBook but can't share that on my iPad. Can anyone help?

Unless it's an app that runs on both operating systems, you'll need to look in the app store under iPad apps to find one for the iPad. I know it's confusing to have different app stores but while iPhone and iPad apps can be cross compatible, to my knowledge macbook and iPad ones aren't.

If the Picassa album is yours, you can view it on the iPad via the Google+ app.

There are several apps compatable with Picassa in the app store.