White Hawk
I'm assuming you've installed K5 and that you've scanned for new plugins or that X3 is auto scanning.
So
Hit B to open the Browser
Choose the tab that is for soft synths
hit the big + sign at the top to add and a synth then browse the menu.
There should be 3 Kontakt 5 in the list.
K5 comes in 3 configurations based on how many outputs each config. has.
4, 8 and 16
but
the name on all of them is the same by default
so eventually you need to rename them is the vst list unless you can figure it out the other way.
You can't add more outputs to the 4 and the 8 so I just removed them and always use the 16 out version.
That should get you started.
There are also plenty of youtube videos about Kontakt 5 so if you want to get the most out of it have a look.