Copy, paste and cut is like for any other app in os x: cmd+c, cmd+v and cmd+x
For trim and join you can either just start typing trim/join like you would do in the windows version and accept with space, you can create your own alias (e.g. tr for trim or whatever you prefer) or you can set up your own shortcuts for Rhino using OS X (System preferences->Keyboard and add Rhino under Keyboard Shortcuts: Application Shortcuts) making for instance cmd+T equal Trim (I’m not sure if OS X will tell you if your new shortcut conflicts with an old one).