SerialForwarder might not start due to a number of different problems.  Let's try to debug:

1.  Is your tinyos-1.x/tools/java directory in your matlab Classpath.txt file?  Go back to "Step 1:  Change your Matlab classpath" in the tutorial and be sure this is typed in correctly.  To test if it is correct, try typing

methods net.tinyos.sf.SerialForward

If you get a list of the methods of this class, the class can be found by matlab.  Otherwise, it is probably a classpath problem.

2.  If matlab can find the SerialForward class but it can't start because it can't find javax.comm, your comm.jar file is probably not in your Matlab classpath.  Go back to "Step 1:  Change your Matlab classpath" in the tutorial and be sure this is typed in correctly.

3.  If Matlab can start the SerialForward class but SerialForward can't connect to the serial port, it might be one of two problems.  Try starting SerialForward from your shell. 

    a.  If SerialForward doesn't work from your shell, then you have a problem with your serial port.

    b.  If it does work from your shell but not from Matlab, it is a JRE problem.  Go back to "Step 3:  Changing Matlab's JRE" in the tutorial and be sure that you changed your JRE.  (The problem with the Matlab JRE is that it has it's own comm.jar file that it looks for. However, this is the GNU version, and we want the javax version.  The Matlab JRE it tries to use the GNU version even if you put the other comm.jar file in your Matlab classpath.) (Note that instead of changing the JRE you could simply start SerialForwarder from your shell.  The rest of the tutorial will go through just fine.)