The JSP decorator feature is not documented in the online decorators guide.
I think if it were, a lot more people would use them.
Ate describes very well what must be done in his comments for this issue:
The JSP decorators can be enabled by modifying (properly commented) settings in:
Note: you need to change the settings in all of the above properties files, and then restart jetspeed.
Also note: for the tcolumns template there isn't a JSP replacement (yet), but that isn't much used anyway AFAIK.
Just to be sure: don't switch your page layout portlet(s) from columns to tcolumns, otherwise your page will be broken.