I use a Mac, but I will assume the procedure is the same in Windows. There are probably other ways to do this, but this is how I put ebooks from
Calibre onto the Kindle app on my iPad:
1. Connect your iPad to you computer.
2. Click on your iPad under Device in iTunes.
3. Click Apps in the main iTunes window.
4. Near the bottom, under File Sharing, you will see a listing of iPad apps that allow file sharing. Click on Kindle. Note the Kindle Documents area that appears to the right.
5. Drag whatever ebook file(s) you want from the
Calibre window and drop them on the Kindle Documents area in iTunes. (I find I need to hover over the window for about a second, or the files will not drop).
6. Click the Apply button, and sync the iPad.