First, don't put any DMGs into Applications. If you have one there now, please remove it.
Next, installing Mac apps is REALLY EASY! Once you know how. So I'm going to tell you how.
Most Mac software is distributed as DMGs. Not all, but something like 75%. If you have a DMG, just do this:
1. Double click the DMG to open it. This is like mounting an invisible CD ROM. It will pop open and (most of the time) show you a license agreement.
2. Agree to the license agreement.
3. How you should see a window showing the contents of the DMG. If you don't, look in the side bar of the Finder and you'll see a disk with the name of the DMG you just mounted. Click it and it will show the contents of the DMG.
4. Many DMGs will have the App and a shortcut to the Applications folder inside their window. They do this so that it's easy for you to drag the app to Applications. Just grab the app, and drag it over top of the shortcut to Applications. You'll see a green plus (+) appear when you're over the top. Let go and it will copy itself into Applications.
5. The app is installed at this point. Now just eject the DMG from the side bar of the Finder. This is just a cleanup step and doesn't affect the app installation.
Now you can run your app from Applications as it is properly installed.
Good luck.
Brian.