how do I down load a ebook that is a android from the net to my imac

If the book is from Amazon, you'll need the Kindle app. You cannot run an android app on a Mac but you can run the native Kindle Mac app. If you have issues, contact Amazon.

ebooks are not formatted in Android. Depending on the format, and how you acquired the ebook, you may not be able to transfer it into books or may need the appropriate reading app.