By default, when you "pull" on the mouse wheel, Google Earth zooms out. When you "push" on the mouse wheel, Google Earth zooms in. To some people, this seems backwards. Fortunately, this can be reversed so that a "pull" zooms you closer and a "push" sends you higher.
To reverse the mouse wheel zoom, click "Tools" on the Google Earth menu.
Then select "Options".
A busy window opens. Click the "Navigation" tab.
Check "Invert mouse wheel zoom direction".
Click the "OK" button to save the changes.