Eclipse plugin doesn't recognize jar files?

Hi,

I have some classes in a jar file that's included in a Scala project.
The jar file is included
in Referenced Libraries. However, in the IDE, the classes from the jar
file seem not available.
Let's say I have this import line:

import com.mycompany.mypackage

The IDE shows error "value mycompany is not a member of package com".

Is this expected?

Thanks,
Jack