How to use JRexCanvas directly?

I'm a complete beginner with JRex, and I want to be able to render 
HTML/XHTML with a little javascript in a sub-pane as part of my program 
(a sub pane in a JSplitPane to be precise). JRex looked a good bet, and 
I was prepared to pay the price of the few Mb needed for distribution 
because of the quality rendering and Javascript support. JRexCanvas 
looked to be the thing to use:
        Runtime.getRuntime().traceMethodCalls(true);
        JFrame f = new JFrame();
        //set the system property "jrex.gre.path" to Mozilla bin directory.
        System.setProperty("jrex.gre.path", 
"C:/Thirdparty_vs7/java/jrex/jrex_gre");

            JRexFactory.getInstance().startEngine();
            JRexCanvas jrc = JRexCanvas.createBrowserComponent();
            JRexWindowManager winManager = 
(JRexWindowManager)JRexFactory.getInstance().getImplInstance(JRexFactory.WINDOW_MANAGER);
            winManager.create(JRexWindowManager.INTERNAL_PANE_MODE);
            
jrc.getNavigator().loadURI("file:///C:/SAF/saf/programs/safadmin/help/HierarchyView.html",
                                       
WebNavigationConstants.LOAD_FLAGS_NONE,
                                       null, null, null);
            f.pack();
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            f.setVisible(true);

and get this error:
...
org.mozilla.jrex.JRexFactory --->**** getImplInstance()-->key 
<WindowManager>
org.mozilla.jrex.window.JRexWindowManagerImpl --->**** create()--> mode<4>
org.mozilla.jrex.navigation.WebNavigationImpl --->**** loadURI 
jrexPeer<0> loadFlags<0> 
uri<file:///C:/SAF/saf/programs/safadmin/help/HierarchyView.html>postData<null> 
headers<null> ****
LoadURI()--> **** juri <0007FA80> loadFlags <0> jrefUri<00000000> 
postData<00000000> headers<00000000> ****
LoadURI()--> **** JRexWindow <00000000> ****
******* ThrowJRexException msg <**** LoadURI()-->YOU DO NOT EXIST!!! 
****> status <0> *******
******* JRex_ThrowByName nameorg/mozilla/jrex/exception/JRexException 
msg **** LoadURI()-->YOU DO NOT EXIST!!! **** [NS-ERROR<0x0L>] *******
org.mozilla.jrex.exception.JRexException: **** LoadURI()-->YOU DO NOT 
EXIST!!! **** [NS-ERROR<0x0L>]
    at org.mozilla.jrex.navigation.WebNavigationImpl.LoadURI(Native Method)
    at 
org.mozilla.jrex.navigation.WebNavigationImpl.loadURI(WebNavigationImpl.java:101)
    at test.HTMLRenderer.main(HTMLRenderer.java:78)

The HTML does exist. I've looked at the JRex source and can see that 
JRexWindowManager.INTERNAL_PANE_MODE doesn't actually do anything, so 
I'm wondering if what I want is actually possible...

Steve