无法添加库到我的Andr​​oid项目

我创建一个新的应用程序，我想用一些图书馆这样的： https://github.com/JakeWharton/Android-ViewPagerIndicator

I'm creating a new app and I want to use some libraries like this: https://github.com/JakeWharton/Android-ViewPagerIndicator

我已经下载了这个库，把它添加到我的工作区。 又到物业> Android和检查是库。 然后我去物业> Android的在我的应用程序的项目，并点击添加对库款，并选择了图书馆，和它给我的库靠近它一个灰色V。 我点击确定，但不能使用图书馆所以我再次打开属性>机器人，并有图书馆附近，和一个红叉？在项目一栏。

I had downloaded this library, added it to my workspace using New Android Project -> Create from existig source. Went to properties > android, and checked Is Library. Then I went to properties > android in my app project, and clicked add on the Libraries section, and chose the library, and It show me the library with a green V near it. I've clicked ok, but couldn't use the library so I've opened the properties > android again, and there was a red cross near the library, and '?' under the "Project" column.

我不明白为什么它的发生！这真是烦人，发生在我身上时，我尝试添加Facebook的API来我的应用程序，我只是放弃了复制源到我的项目。

I can't understand why it's happening! It's really annoying, happened to me when I tried to add facebook API to my app, and I just gave up and copied the source to my project.

谢谢，埃拉德！