Look at the Java EE examples shipped with HornetQ 2.2.5. I know there are a few that have modified the ra.xml. Aside from that, your capitalization looks wrong. Try this:
<description>The number of JMS sessions to allocate for message consumers</description>
Lastly, I don't think you would be able to set this on the MDB class even if you wanted to. If you are, as you say, not using EJB3 then activation configuration properties like this would have be set in ejb-jar.xml (or perhaps jboss.xml). Furthermore, you could use system property substitution to externalize any of these values so they could be easily changed for different environments.