Piergiorgio,
The Activiti BPMN Designer is outdated and it cannot be installed into a modern version of the product like MyEclipse 2015. MyEclipse 2015 has greater version of graphiti than the Activiti designer requires. Please contact the Activiti team about updating their designer.
While we recommend working with MyEclipse 2015, our latest release, the Activiti BPMN Designer can be installed to MyEclipse 2014 using the update site : http://www.activiti.org/designer/update.
Go to Help > Install from Site > Add. Give the update site location. Make sure the ‘Group items by category’ is checked. Select the ‘Activiti BPMN Designer’ and follow the wizard.
You can verify if the plugin is installed successfully from Help > About MyEclipse > Installation Details > Installed Software.
The MyEclipse 2014 installer can be downloaded from here : https://www.genuitec.com/products/myeclipse/download/
Let us know if you have any questions.
–Swapna
MyEclipse Support