In java actions you can only change the code between the comments "begin user code" and "end user code". If you edit anything outside of that, the modeler will overwrite it. Whats the error that you are receiving? Is your email module up to date? Maybe try updating it from the app store?
Edit: Oh I've seen this before. Sometimes the module will add "Parameter1" at the end of parameters when importing. When you remove the parameter 1 part, the modeler overwrites these changes? Check the java action in the modeler to see what the parameter is called. If it has parameter1 in it, you can change it from the modeler.
I have just downloaded the EmailTemplate module from the App Store.
When i try to run my application it get errors related to java action SendMail.java:
It is easy to spot the error and correct it
The highlighted line should say
this.Subject = SubjectParameter1
because on 3 plaes there are references to this.subject
I have changed the 3 references as you indicated, and that indeed works.
Thanks for your excellent support.