geek.shrek,
MyEclipse doesn’t provide an integrated support for cactus plugin. I would recommend you to do the following, check the compatibility of catcus plugin with that of your base eclipse 3.4.2. If it supports, you can add cactus as a plugin to MyEclipse. If you have an update site, you can add the plugin from MyEclipse by going through Help > Software Updates > Add/Remove Software.
If you have a .zip file with plugins/features. You can unzip the same to [your MyEclipse installation dir]/dropins.
Hope this helps.