When you are iteratively trying to correct a mapping it makes a lot of sense to create a little batch file which can automate invoking msgtransform and msgdiff. Then you can make a change in Chameleon, save your message definition file, run the batch file and then compare the output very rapidly. For instance the following batch file works very well:

msgtransform hl7tohl7.vmd before.txt transformed.txt
rem oldtransform.txt is the transformed result from another transform
msgdiff transformed.txt oldtransform.txt before.txt