There's a required structure for the pathways where templates are stored.
First, you need to be inside the User Folder that you use FCPX with. If you only have one user on your system, then this is easy. From the "root" of your internal drive, find the Users folder and double click on it. Inside you should see an icon of a small house ("home"). Double click on that. Inside is the Movies folder (automatically set up for every user).
If you don't have Motion, or you have never installed templates before, then you will need to create a Motion Templates folder (if it does not already exist). Right click on that folder and Get Info. In the Get Info window, find Name & Extension. Make sure the text in the box is: Motion Templates.localized and close the window.
Double click Motion Templates to open it. There should four folders inside: Effects, Generators, Titles and Transitions. If any or all do not exist, create them. For each one, repeat the "localized" step above. These are the only folders (along with Motion Templates) that need to be localized.
For FCPX to function properly, the next level of folders are "category" folders. The names of these folders correspond exactly with the Categories that show up in FCPX for each type of effect, for example: Basic, 3D Titles, Solids, etc. Basically, you want to create categories that will make the effects you are looking for easy to find.
It is inside the Category folders that Templates are placed. You follow the correct path for the type of template (Effect, Titles, etc., then category).
A Template is the entire folder containing the Motion project (.moef, .motn, .moti, .motr file), plus two PNGs ("large" and "small" — thumbnails) and a Media folder (these are minimum, there can be more!) The Template folder is the exact same name as the Motion project file inside [example: "Basic Title" folder and "Basic Title.moti" Motion project].
If anything in this pathway is "out of whack", FCPX will not show the template or it will show it, but with a "Missing media" icon, so everything needs to line up exactly. The most common mistakes are the ".localized" extensions missing and/or the Category folder "level".
There are two other locations where templates can be stored (that I know of), but neither one is recommended by Apple for third party installation. However, if you need to install a template for "all users", then look to
Macintosh HD > Library > Application Support > Final Cut Pro > Templates >
(equivalent to the Motion Templates level above - same rules for subfolders applies from here [including "localized"].)