Provide the path upto the jar file name.
Check below code. I coud able to see the Logger class loaded of slf4j
ClassPool pool = new ClassPool(ClassPool.getDefault());
pool.appendClassPath("./otherlib/slf4j-api-1.7.6.jar");
CtClass ctClass = pool.get("org.slf4j.Logger");
System.out.println(ctClass);