Why won't Eclipse/adb detect my Samsung Galaxy Tab device on my Mac?

Make sure you uncheck

• Settings > Applications > Unknown Sources (Allow installation of non-Market applications)

Turn on USB Debugging inside Settings > Applications > Developers Settings

If you're trying to connect to a Mac, I had this issue with a Samsung Galaxy Tab 10.1 running Android 4.3. To enable developer options, open the Settings app, tap on "About device" under the "General" tab and then tap on the "Build number" repeatedly (it's grayed out but you can tap on it anyway). A small pop-up will appear showing the amount of additional taps you need to do to turn on these options.

Once enabled, just turn on USB Debugging and it'll show up in Eclipse just fine.