If you're talking a special, unique transition that you can't find in the effect browser, you have to use Motion to create the transition yourself, and publish it to FCPX. If you're just looking for a slow dissolve, there's already a dissolve transition available. Just drag it from the effects browser to your edit.
Edit: Here's one I created in Motion. Delete the .xml extension that google adds to the filename; it should be called "SideBySideGradient.motr", not "SideBySideGradient.motr.xml" Put this file at ~/Movies/Motion Templates. If you don't already have a Motion Templates folder in your [home directory]/Movies, create one there, drop this in, and look for "SideBySideGradient" in your FCPX effects browser under the transitions category. Drag this transition between two clips on your timeline, and VIOLA!
I realize that this is a pretty simple application which might not suit your needs exactly, which is why I encourage you to use motion and try it for yourself. It's not hard! If you need help, feel free to ask.