Depends on their format. Are you using a specific 3D package? If not, something like Blender would be a good place to start if you need to convert them.
If you're using royalty free models they usually come in OBJ format and will *usually* go straight into UDK. You open the Content Browser and click Import. There are many many tutorials on static mesh importing so I'd recommend you check those out first.
note that IF you use them, make sure you manually save the package, otherwise your contents are gone. Use an easy-to-find name!
Not to mention, not that in some versions of UDK, the materials used on your custom mesh can all of a sudden fail! If that happens, don't hold yourself from asking for aid here. I'm still not worked out with that issue as well.
Make sure you are using the 2013-07 version of the UDK editor or it may not display the meshes properly due to the fact that it uses materials intergrated within the editor itself.
The meshes are free also, free to use free to modify. All that I ask is a link to my site on your finished product. You don't have to but it would be appreciated.