To Enter the Custom Transition Effect Window click on the AB symbol in a slide or:

Click on Project Options / Transitions

Then click on the Manage Transitions button

Give the Transition a descriptive name

Choose an existing Category or Type in a New Category Name

Next, choose a Variant

E.g. Multi Variant - List

Click on “+” to add a Variant

Click on “-” to delete the selected Variant

Click on “Rename” to give the Variant an appropriate Name

Click on “Edit” to enter the “Editor of Transition Effect” Window