我想你是使用Eclipse。你的活动是有好的，但是Eclipse没有包括的支持包中的APK ......如果你看看上面的堆栈跟踪你的错误，你会发现你的问题的根源：找不到FragmentActivity 。或者你可以解开APK，其水下爆炸，你会看到不包括兼容包中的类。

I suppose you're using Eclipse. Your activity is there alright, but Eclipse didn't include the support package in the APK ... If you look above your error in stacktrace you will notice the root of your problem: "Unable to find FragmentActivity". Or you can unpack the apk, undex it and you will see the compatibility package classes were not included.

要解决此问题，用鼠标右键单击您的项目，属性，构建路径，去出口标签。有确保 Android的私人图书馆进行检查。干净，重建应该把你的轨道上......

To fix this, right-click your project, properties, build path and go to export tab. There make sure "Android private libraries" are checked. A clean and rebuild should put you on track ...