Alex,
MyEclipse 2014 is bundled with org.eclipse.ui.ide version 3.9.1. MyEclipse 2015 is based on Eclipse Luna and comes bundled with org.eclipse.ui.ide version 3.10.2.
You can go to menu Help > About MyEclipse Workbench > Installation Details > switch to Plug-ins tab and type org.eclipse.ui.ide in the filter text to check the version of the plugin.
While installing third party plugins to MyEclipse, please ensure that the plugin version is compatible with the base Eclipse version on which MyEclipse is built. MyEclipse 2015 is based on Eclipse 4.4.2 (Luna) and MyEclipse 2016 is based on Eclipse 4.5.1 (Mars).
Please download the MyEclipse 2015 installer from here : https://www.genuitec.com/products/myeclipse/download/
You can install MyEclipse 2015 to a different location alongside your existing MyEclipse 2014 install and use your current license.
If you are still seeing issues installing the plugin in MyEclipse 2015, then please share with us the update site details of the plugin to help us investigate further.
Please let us know how it works for you.
–Swapna
MyEclipse Support