All of this is in the preferences, which you can open by pushing the keyboard shortcut "command-," or by going up to the menu bar and selecting "MixEmergency" and then "Preferences...". The preferences window will then open on the General panel - and on this window you can split the preview windows. For the transitions you will see that there is an icon on the preference window title "Transition" (from here you can select which transition are included in your "Random" transition list) and also an icon with the title "Plug-ins" (which allows you to select and rearrange the effect and transition plug-ins that are available to you in your effect and transition lists).
