I extracted your zip file. There is no icons folder in src/main/resources/ . There is also nothing in the icons folder at the root of the project. But that's not a Maven resource folder anyway, so so should throw that one away. You can also verify this yourself, since you included the target directory, which is Eclipse's output folder for Maven projects. There's no icons folder in there.
I just launch activiti designer from my Eclipse development installation and link to your jar to test it. You could also install a separate version of Eclipse, install Activiti Designer from the update site and then link it. Makes little difference, other than from the dev installation you can inspect errors and debug code more easily.