You should really have read the FAQ, the answer is in there, regardless...Hibernate needs some third party libraries before it can run. This is one of them, go here
http://www.findjar.com/jar/milyn/jars/j2ee-1.4.jar.html
And look for the SLF4J JAR, download it and put it in a directory in your java project path. Then open the Java build path menu in your compiler/IDE and add the new JAR as a dependency (in Eclipse just right click on the project, go to properties > Java Build Path > Libraries > Add External JARS.
That should fix it right up, you may have to add another couple libraries though.
Alternatively, look into a build manager called Maven, it does all this for you :D