If you want to watch a DivX video on your iPad, download oPlayer Lite from the ipad’s App Store. An iPad can’t open a.dmg file.dmg files are used for installing applications on a Mac and only work in MacOS.
You probably already have it if you're a Windows regular, since it's a free and convenient way of extracting the more universal ZIP format. The best option, whenever possible, is using 7-Zip. Here's how to extract content from a DMG in Windows 10. One of the potentially frustrating things about switching back and forth between Mac and Windows PCs can be proprietary formats - a key example being Apple's DMGs (disk images).