It's looking for the javahelp jhall.jar file on the system and can't find it. What version of SUNWjhrt do you have installed? It might be that imqadmin can't find jhall.jar's newer locations with their Version 2.0.
If you have SUNWjhrt Version 1.1.X it's going to look for jhall.jar in: /usr/j2se/opt/javahelp/lib/jhall.jar
If the jhall.jar is in another location, then you have a newer version of SUNWjhrt so a workaround would be to copy jhall.jar to the directory /usr/share/lib/imq and restart imqadmin.
BTW, we fixed this problem in MQ3.6. Thanks.
http://wwws.sun.com/software/products/message_queue/