Cannot install custom transitions

Please post a screenshot of the whole hierarchy of folders, in Column View, from Motion Templates down to one of the transitions you are trying to install.

Two of the common issues are: just putting the .motr file instead of the whole folder containing it; and not placing this folder inside a “category” folder. The category can be named anything meaningful, it will be used for grouping related transitions in tge Transitions Browser inside FCP.

Here is an example of how the folder hierarchy may look like.

In this example, "Lichinho" is the name of the category I used.

Did the transitions have an installer? Who's the developer? What were the developer's instructions? Do you have Motion installed? Did you .localize the appropriate folders?

Have you tied shutting down FCP and your Mac after installing them?

This can often kick-start FCP into "seeing" them.