Did you take a look at whether the jar file actually contains that class?
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
Don't confuse the classpath for Ant itself with the classpath for the taskdef. You probably need to add a classpath definition to the task. For some tasks, I've actually had to do 2 classpaths - one for the taskdef itself and one for the defined task's execution.
An IDE is no substitute for an Intelligent Developer.