In recent versions of MyEclipse, the Eclipse Marketplace may not be accessible.
To fix this, edit the config.ini file located in the MyEclipse installation directory/configuration folder and modify the com.genuitec.pulse.delivery.mpc.include.patterns entry to use https as shown below : com.genuitec.pulse.delivery.mpc.include.patterns=^https://marketplace.eclipse.org.*
Once done, restart the IDE to apply the changes. You should now see the Eclipse Marketplace entry in the Help > Install from Catalog dialog.
This is caused due to the fact that the Eclipse Marketplace API now requires HTTPS.