Hi Sean,
quoting the user manual:
1) Choose the Select tool from the Tools pop-up menu in the toolbar and click the edge of a clip (an edit point) in the Timeline.
One or both clip edges are highlighted.
Note: If you select a whole clip in the Timeline instead of just an edit point, transitions are added at each end of the selected clip.
2) Choose Edit > Add Cross Dissolve (or press Command-T)
this is on page 206 of the manual.
I would recommend you check it out when having questions regarding the basic fundamentals of FCPX. Nothing wrong with asking here but the manual is a well written, SEARCHABLE pdf document.I am quite sure it took me less time to find that info than it took you to login and write your question here 😉
Further info about transitions in that chapter, including applying multiple transitions at once.
hope this helps