How are you adding your Weblogic classes to the classpath in Eclipse? Are you using WTP, and a server runtime? If so, is your server runtime associated with your project?
If you right click on your project and choose build path->configure build path and then choose the libraries tab. You should see the weblogic libraries associated here. If you do not you can click Add Library->Server Runtime. If the library is not there, then you first need to configure it. Windows->Preferences->Server->Installed runtimes