How can I change the implementation default value? can you give an example?
Here is an example of the source code that the custom service task (workitemdefinition) must have:
<bpmn2:serviceTask (...) name="My Task" implementation="#CustomValue" (...) >
I think this isn't editable in designer, see
"NOTE: Unfortunately tooling does not support this yet so the bpmn2 file needs to be edited by hand. Soon tooling will provide this as well."
That note is a little outdated - we have added support for it in Designer (http://i.imgur.com/G18FJzu.png) for service tasks, and imports. This is available to test in Designer master github, give it a try if you want and report any issues you may find.